Starting off with the easy one.
Unit: Firebat (or Marauder)
Above avg Hp, Avg speed, avg damage.
Niche: Ability to build tons of stuff that can be used to defend, control, or assault. Versatility. Weakness comes from lack of spells that do major damage.

Spell 1: Defensive Turret
Spawns just like normal Defensive turrets, except give it a 1-2 second buildtime (doesn't require Engineer to build along with it)
HP - 120. Damage 10 + 1.5. - 40 second lifetime

Spell 2: Build
What things should be allowed to build?
Here are some suggestions.

1. Warden (for lack of better name)
- Ripping TF2 for this one. A building Heals (slowly) the Engineer, allies, and Defensive turrets that're close to it.
The healing effects should Stack, and we can make there to be max 3-5 wardens

2. Sapper (for lack of better name)
- Exact building from the Single player, where it slows down enemy units when in range of the building

3. Ramper (for lack of better name)
- Same as Warden, except it replenishes mana when the Engineer is nearby.

4. Barricades
- Supply depots that can be lowered. There should be a way to make it so that spawns can walk straight past it but not heroes. I've seen it somewhere before but I'm not a hundred percent sure if this is doable or not.

Spell 3 - Build [insert cool tank name here]
(some Tank names - Armored Fist - Thunderer - Challenger - BT2 Blitz - etc. etc.)
- Engineer builds a tank - We can use the effects from building a normal terran building and switch it to a tank
- The Engineer needs to be loaded into the tank in order to operate it.
- 500 HP 10 Armor - dmg = 45. When Sieged = 90 + splash
- make its speed low and its attack speed very low.
- Killing the tank should hurt the Engineer for 500 dmg.

Spell 4 - ????
Sorry, still haven't figured out a good L4 for him yet

Again, the assault in TS1 isn't that well defined. I have somewhat reimagined it with the current spells and mechanics
Unit: Reaper
Speed - Fast - Attack speed fast. Hp below average
probably a starting HP similar to the Mutant, but a bit beefier, with damage similar.

Spell 1 - Afterburn
Increases Reaper's speed dramatically, and also his attack speed, for 4-6 seconds.
The effect needs to feel short but efficient.
It should also allow the Assault to jump up and down cliffs (screw the only jumping down cliffs, it'd just confuse the players)

Spell 2 - Null Blast
So I've thought about this, and I think the assault is better suited with the Null blast. It's more befitting than the Tear Gas, which pretty much does the same thing. Also, since Null Blast is complete, we can put it here.
After playtesting this spell, I want to make these following changes.
- 50% attack is very hard to notice, and doesn't seem that important in the thick of battle, especially the way the heroes interact atm. So lets drop it and just keep the 50% speed, because it synergizes very well with Reaper's speed.
- Add a really big Knockback effect. Like maybe 5 times the knockback effect that's on LM's current L2. The Blast doesn't do damage, but the knockback gives the blast its well deserved weight. Plus, it synergizes again, as clever players can use this to bounce players backwards, allowing them to land a few hits. Moreover, it moves spawns away. It might as well give allied units the knockback effect as well (but not the negative effects, obviously)
- Let's increase it's missile speed. It is too instantaneous. Let's make it have a 1.5 second missile speed.
- It's range might need an increase. Not too sure.
- Finally, what is its duration atm? I think 6 seconds is good.

Spell 3 - Cluster Bombs.
Yep, Luzz's reimagination of Spec Ops/Engineer's L4 should go here.
It is a perfect place to put it because it the spell isn't as flashy as it's TS1 counterpart (and it really shouldn't be) and not as crazy as calling down a dropship to drop bombs onto the battlefield, but it's usefulness and destruction makes it a prime L3.
Why the assault? Because Spec Op's Calldown UAV has taken its place, and it serves well there.
- Initial Explosions should do around 180+8 damage each (180x3, if the hero happens to standing in the middle of all three, tough luck)
- We can keep the little mines as the way they are.
- Now, let's make it so that the initial explosions do extra damage to buildings. like 60 more damage (so ~240 x 3 = 720). Yep, this spell has found it's niche in the Assault's spells category as the building crusher.

Spell 4 - Aerial Assault
The dropship that drops bombs and etc.

The unit and it's damage output can stay the same.

The big concern are its spells. Spider mines is kind of a crutch as a first spell. It doesn't really synergize, it's not a very active spell, can't really gank people, can't really help you escape.. and etc.
So, if we were to replace it, here's what I'd suggest

Spell 1 - C4 (for lack of better name... srsly this time)
Spec Ops plants a mine at some distance away from him. Multiple can be placed at one time.
- another button detonates all of them. Explosion + small knockback and maybe a short stun time?
- Deals 70+7 damage.
- We can make the c4 invincible, but small, and usually easily avoidable. A small lag time between pressing the button and detonation is a must!
It just feels like C4 would be a more fun variant. Tho I admit it doesn't really synergize with anything either, but it gives the player more control. Moreover, if we add effects like a short stun, it might help in a tight game

Spell 2 - Calldown [insert Gadget name here]
- Calls down OR Have the gadget decloak next to him.
- it is a small robot (i'm thinking either a Mule, or that cute little robot critter) that has 3 abilities
- It has 50 HP, but very fast speed. No normal attack
- Has it's own Energy line.
1. Self-Destruct - no E - more damage than C4 (around 150+10) and stun for 4 seconds. Bigger AoE and bigger explosion
2. Discharge - costly E - The robot gives an electric discharge, shocking and stunning enemies around it. effects can be any of the follow, draining mana, stunning, or slowing them down.
3. Cloak - channeled, slowly drains energy

Spell 3 - Calldown UAV
- Detector that allows irradiation. Brilliant
- I think we should add Scanner Sweep onto the UAV. We can make it more E costly.

Spell 4 - ???
Unfortunately, not sure about this one yet

The current Unit and damage is fine.
The main problem with the current volt (besides not having a Spell 2) is that its spells do not have the right feel to it. They lack weight but also lack... well.. electricity.
We cannot simply copy TS1 for this hero because the only reason TS1 volt turned out the way he is is due to SC1's limitations. We can definitely do better than that, while keeping the same sort of gameplay mechanics and experience people get from TS1 Volt.

Spell 1 - Shockwave
The current shockwave is lacking quite a bit, both in effects and mechanics.
Focusing on mechanics, it needs to have a bigger AoE and probably a bit more damage (I think 20dmg is its current? Let's make it 40+5)
It should be made to repel and stun enemies, thinning the crowd for Volt. It's stun time is fine, but it needs a bigger knockback.

As for effects, I think that we should make the current Volt's L3 effect into L1, giving it a small charge time (like a second or something)
Like Null Blast, I want the Shockwave to have very good knockback/knockup, able to spread units away, giving it more use and better feel.

Spell 2 - Magnetic Channel
- Volt shoots a projectile in direction of target. Projectile binds to first enemy unit it encounters.
- The range of the projectile should be fixed (i.e. the target only specifies the direction of the projectile, not where the projectile will stop)
- Once bound, the unit and Volt is connected with a beam (probably Void Ray level 1 beam), which pulls the unit rather quickly towards Volt.
- The Unit will be bound for 3-5 seconds (needs to be balanced)
This spell becomes a Skill Shot (much like Meat Hook from Pudge from Dota). The reason for this is I feel that Volt already has too many AoE attacks, and needs something to synergize with it.
Also, now that Spell 1 spreads units and stuns them, it makes a great combo with this spell.
I am aware that it may be too similar to other mechanics, but we can always tweak it with different ideas.
As for how in TS1, this spell makes a lot of other spells not work.. well, we can always tweak things to give mana drain.
**I am not sure how this should work with the Cliffs. It should probably work over cliffs somehow.

Spell 3 - ???

Spell 4 is fine. Just needs a buff

For engineer, I think SCVs should be kept for L2 (maybe change them to hovering drones)
This way he can repair and build across that map, without having to run back and forth too much, thus losing out on training time. They could also repair the tank. Shouldn't engineer have armored refineries? —and if he does he doesn't seem to have anything to hold that ground as turrets only last 40 seconds. He should either have a type of turret that doesn't gain exp from killing spawn, that lasts permanently, or have a specialized permanent high-dmg turret that only targets enemy heroes and their summons. OR the turrets can have an upgrade function to turn into those and other specialized forms (anti-air turret, siege-mode turret). As for the warden & ramper, I think they should be consolidated into one building (players will just build them next to each other anyway). Perhaps just add an upgrade feature, or mode-toggle between mana/hp. Also, it might work better if the ramper/warden gave mana/hp very rapidly, but had a supply limit (separate per individual team-member) that slowly regenerates. This way, it tends to keep a faster pace, by just making quick-stops to put up supplies.

If you decide to leave out SCVs, then make the turrets viable enough to somewhat train a lane (without getting pwned by heroes) while he runs the map trying to build stuff. For a variant of the sapper, you could have an "Arc Tower" or "Tesla Coil". It sends a bolt at nearby foes from a relatively large distance. The bolt does minor damage, but has a long slow time (7 seconds?), and a cooldown just as long (spawns might even be attacked at a quicker rate). I assume the sapper is just an AoE device, whereas this has a lasting effect. If a player just enters the sapper range, he can run out and flee— but with the Arc tower, a player struck can only crawl away slowly, and the effect will still stay if they take down the device too quickly. This would work better for map defense against single enemies, but weak to group attacks.

L4 can be Contruct Golem. A heavy, high-dmg, very slow summon. They wreak bases, spawns, and heroe's minions. However, their slow speed makes them easy to intercept by enemy heroes.
The golems could possibly have upgrades to make them more specialized.

I think it'd be more interesting to see Eng as something mechanical, rather than another human-type (assault, spec ops, medic, phantom, warrior? dark mage? light mage?) like a medium-size unit Mule or SCV, but with splash med-short ranged attack.

Maybe it's just me, but feels like too many drone gadgets.
L1 should be able to selectively explode a charge as well as detonate all (ability to highlight singular or multiple).

L2- Tactical Weaponry:
Ability to choose between specialized weapons for a limited time. For close-range you can pull out an auto-shotgun. Medium range you may use a grenade launcher. Long range, a semi-auto high-powered rifle. You may try to position yourself and watch your enemy to optimize your attack. Could possibly work well with the set-up positioning of C4s.

L3 - Predator Drone:
EMP - fires an EMP missle that drains energy/cloak, removes 200-300 shields off heroes, disables building shields, and temporarily disables nearby buildings (enough to slip by and cap or do a sneak attack).
Surveillance Scan - Expands sight over a LARGE area around the drone and highlight foes as well as give a readout on their hp/mana/energy/and other status.
Irradiated Tracker - Fires a tracker dart that reveals the only the unit (not it's sight) as it walks into fog for your team to see, as well as displays it's hp next to him, and deals slow damage. (Think irrad + parasite)
Stealth Mode - cloaks the drone (energy drained)
Maybe defense matrix.

L4 - Nuclear Strike: You'll be sitting there vulnerable for a while, waiting for that nuke to fall... but at least you are watching your back with that surveillance drone, and some C4 to back you up. If worst comes to worst, you can just cancel and take your mana loss. Best part is, the enemy temple won't die in 2 nukes...

L3 - Shock Storm - Large warmup, then instantly drains (150?) mana. Creates an area that continuously deals damage to foes, negates armor, they can't do normal attack, and move EXTREMELY slow. Foes that enter don't move as slow or take as much damage, than foes initially struck. Good to follow up after you pulled them in with L2. Won't kill foes (hp stops lowering at 1) Also keeps the old chain of spells L1 -> L2 -> L3 -> L4.
Make sure L1 clears spawn too.
A natural ability could reduce his shields for a bit while he does a constant burst of his normal attack, followed by a cooldown.