The hunt begins here!
First try, I followed the idea of "Omnitron-X incapacitates Omnitron via Self-Sabotage, then is incapacitated himself before the heroes win (i.e., there needs to be at least one other Device in play). Unity must be on the team." Didn't work.
Notably, Omni-X played all his components before unleashing Self-Sabotage. (It's worth noting that Twist the Ether on him was a huge help in making sure he didn't kill everything too fast.) Unity beat him to death with her bots. I was playing in Enclave of the Endlings. Omni was last in the team order.
Current thought, though it would be unprecedented, is that it might be necessary to use Omni-X's incapacitated power on Unity, maybe letting her get a bot into play before victory?
I predict that it will take longer to figure this out than it took before Bunker was revealed.
It might be possible that this will require the Omnitron IV environment...
The story challenge for Omnitron-U can be completed with the content currently available in the game.
They wouldn't do that to us.
Edit: Although they would post before us, making our posts somewhat obsolete 
So, here's a question where an "official" answer might be useful:
For unlocks (in general), do steps completed before the unlock is in the game count?
So, if step 1 in getting an unlock is "beat Spite in the living room with a candlestick", and you did that six months ago (way before the unlock was in the game), would that count? Or do you have to complete all requisite steps fresh once the unlock challenge is in the game?
I'm asking because I suspect that we might get quite a few false positives on the road to the truth, given that OX, Unity, and Omnitron have all been in the game for a while, with plenty of opportunity to fulfill any theoretical first step that involves them!
They have required you do the steps since they became available in the past. This is the only one that didn't come with a major release though.
No, no, yes.
Added a note about turn order. Someone brought that up last night.
Interesting! Thank you!
Just a side note, but in Omnitron-U's story, didn't Omnitron-X die after COSMIC Omnitron went down? I'm trying to remember the order.
Edit: Think I'm wrong actually, but someone over on the Steam forums does have a screenshot of an unlock.
Cosmitron's Bio states that Omnitron was defeated by the F5, so it seems like OX sacrificed himself to defeat Cosmitron. Though there is probably room for interpretation, what with time travel shenanigans.
my steps tonight will be:
Defeat Omnitron with the F5 in megalopolis
Defeat Cosmitron with OX in lead position, using Self Sacrifice.
(If no unlock)
Defeat Miss Information with Unity in lead position, with Tachyon, Wraith, and Bunker. I choose Miss Information because she's the closest I can approximate to Freedom Tower with.
I'm guessing that it's something like:
- Beat (some version) of Omnitron
- Have a game where O-X dies (maybe the same with Omnitron, maybe not)
- Play a game with Unity where she does.......something
The thing that makes it tricky is that in the lore, O-X survives the battle with Omnitron and then self-terminates, so should that be represented in the game as winning and dying in some other game, or dying in the game where you beat Omnitron?
My impression was that he sacrifices himself to destroy Omnitron. My guess is, you have to use Singularity or Self-Sabotage to damage Omnitron, or have OX get taken out by Technological Singularity. Or maybe win a game against Omnitron where all three of those cards get played?
I wonder if Tachyon is involved somehow. It isn't stated that she helped rebuild O-X, in fact she didn't even find it a worthwhile endeavor. But perhaps she's also required in the team setup in the theoretical third step.
the Omnitron fight keeps going after the boss is defeated as long as there are still drones in play. maybe something like: OmniX self-destructs to kill the boss, then the drones finish OmniX, then the remaining heroes clean up for a victory…? complicated stuff.
The unlock picture did NOT include Tachyon. My guess is that due to her fears about the project, she would need to NOT be there for the 3rd step.
Didn't the unlock show Ra, and no sign of Omnitron-X? Or, at least, the one I saw on Twitter did.
Did somebody unlock it already??