He dose not not have his powers since he is in a teenage girls mind
The emotion he tolerates the most is disgust
There is something so funny yet so thought provoking about putting characters from another show that DO NOT fit at all with Inside Out and still make them believable by adapting them.
In this case, Omni Man would be a good fit because he goes through an insane emotional turmoil which would be fun to explore in Inside Out terms. Like I said, there's something charming seeing a violent character in Inside Out.
As for headcanons, I think Omni Man would pretty much hate everyone, judge the emotions, maybe not caring at all because Riley is a lesser being in his eyes. I think he would retain powers, otherwise he wouldn't be the savage Viltrumite we know and love. Or hate.
I would love to see a fanfic where the emotions have to deal with the threat of Omni Man inside Riley's mind
Anxiety: WHY DID YOU MAKE ME DO THIS?! You're fighting so you can watch Riley be unhappy. THINK, JOY!!!
You'll outlast every single insignificant emotion in Headquarters. YOU'LL LIVE TO SEE THIS MIND CRUMBLE TO DUST AND BLOW AWAY! WHAT WILL YOU HAVE AFTER 90 YEARS?!
I think Envy being "evil" would make her character so much more interesting than just going "Aww" or "Wooow" the entire time. That wouldn't even be Envy, it would simply be admiration.
I truly hope Pixar doesn't waste this golden opportunity to make Envy a memorable character, if she had a bad hidden side, that would be so nice to see. I really don't want a child copy of Joy