Maybe the water doodad is a Xel'Naga construct?

The dirt doodad looks like a human plane to me from the modern-ish era.

Yea, agreed or maybe another Xel'Naga ship o.o

The first one looks definitely like a mech of some sort hunched over in the water, and i always thought the second one was a crashed giant helicopter.

The unit that the mech doodad is is shown on the background of nearly every single page in the SC manual.

maybe it has to do with something special that they're trying to give us a hint about.

It's the right arm plate of a Marine Statue..
Or maybe the Ganithtrithor?

Right. Statues of giant marines were constructed on random planets.

The crashed ship has its first experience in terran03 (survive 30 minutes). In this mission that area around that doodad was used to be a another escape area, if you win the map by killing the Zerg Hatchery (objective was disabled in triggers). You are on the planet Mar Sara.

The robotic doodad has its first experience in terran 07 (the skipped one where you controle duke with tank and raynor and the protoss appear in the end and you controle a protoss fleet.).

I can say without a doubt the doodad in the OP is an Armored Core from the Armored Core series. What seems to be its shoulder piece looks exactly like a off-hand shield used in the Armored Core games.

I even know the model number! ZES-99-MIRROR

http://acfield.fc2web.com/ac2/mission_parts.html#mp10

EDIT: Oh yeah, bump.

Maybe they're just random ass doodads like the rest and arn't moddled after any sc unit/ship.