The UED from Brood War sent a tiny expeditionary fleet to the Koprulu Sector to conquer and liberate the Terran Dominion (Similar to Matt Horner's and Raynor's goals in revolution, right? Except instead of conquer, it's revolution), and to take control of the entire Zerg Swarm under the Second Overmind. With the Zerg under the UED's controls, the UED seeks to pacify and seek peace with the Protoss. Which is similar to the Umojan Protectorate's goals in peace. The UED's goals were met, regardless of Duran's manipulation. The remnants of the Dominion, Raynor's Raiders, the Protoss, and Sarah Kerrigan's Swarm were aliied together to temporarily to stop the "tiny" UED's onslaught. The Second Overmind, under UED's control and drugs, is destroyed. Somehow relinquishing the entire Zerg Swarm to Kerrigan's control. Kerrigan betrayed everybody, and the chaos between everyone in the Koprulu Sector remains the same as before the UED's arrival. The Dominion rebuilds, the Zerg hides in Char, the Protoss recovers in Shakuras, and Raynor turned poor. However, the main goal of the UED is to establish peace between every faction in the Koprulu Sector. The people, and the full force of the UED at Earth lost interest in the Koprulu Sector and were like "meh, maybe later". Remember, the UED fleet in Brood War is very small, they have degraded tech compared to what they have back home.
Now let's compare the United Earth Directorate and the Xel'Naga.
The UED and the Xel'Naga share a similar goal: Conquer the galaxy.
The UED is very powerful, as the reporter in the UED Victory Report in Brood War boasted how they have "highly trained directorate forces", and has the "greatest military technology in the galaxy". Their goals in the Koprulu Sector is to keep peace with the Protoss, control the Zerg, and own all Terran factions such as the Dominion, Umojan Protectorate, Kel-Morian Combine, and the Confederacy (Which fell defeat by the Dominion). But it failed. The technology of the UED will always surpass the Dominion, regardless of the Dominion's experimental weapons such as the weapon in the secret mission where Raynor is able to take down the Brutalisk in several shots.
No one knows what the Xel'Naga look like, but people such as Raynor know the power of the Xel'Naga artifact. The blast of the Xel'Naga artifact eradicated every possible Zerg on Char, but not as powerful as the Xel'Naga Temple's blast in Shakuras (Though the Zerg came back to Shakuras anyways). At the end of the mission "In Utter Darkness", the Hybrids simply unleashed their blast to eradicate all Zerg after the Protoss's demise. So expect something in the Zerg campaign that if fighting the hybrids, they would probably rape your forces with a blast like there's no tomorrow. The Terran infantry in the secret mission cannot even damage that lone Hybrid. The Hybrid, already self explanatory, is powerful even in in groups. Expect new Hybrid units in the next two expansion. The Protoss, being the last and final story in StarCraft II, has the power to kill the Hybrid with their psi powers. With the help of (half of) the Dominion fleet, Raynor, and Kerrigan's Zerg, they would stop the Hybrid. But Kerrigan alone is the key to stop it.
UED owned the Zerg as slaves, and absorbed defeated Terran forces into the UED fleet.
The Xel'Naga's hybrids owned the Protoss slaves, Zerg slaves in the Overmind's vision, and Dominion scientists helped create hybrids...
So I believe, everyone mentioned will team up against the powerful, again; except the ending will be a more peaceful aftermath (Or not, which could help bring StarCraft III in another decade or so). I also believe the other half of the Dominion fleet (I mean, why the hell is Arcturus Mengsk involved creating the Hybrids seen in the secret mission? What an asshole), several Protoss and Zerg slaves will work for the Hybrid. Just like how in WarCraft III Reign of Chaos: the Alliance, the Horde, and the Night Elves teamed up to defend the World Tree against the Undead (Being slaves) and the demons. Except far more powerful.
Thoughts?
None.