Inspired by Duty Bound by agarwaenloth (on AO3).
Semi-AU. All the Avengers (+ Phil) were basically forced under SHIELD's control. Hostages are taken, kept in unknown locations. Each Avenger is kept locked up in individual cells between missions (and maybe Tony's arc reactor is removed and they have him hooked up to some kind of machine by cables in the walls to discourage escape attempts). Fury works them to exhaustion, almost. And the thing is, Fury is certain that this is the best way to handle them. That this is the only way he can be sure they will follow orders when the world needs saving.
Other superhero teams are completely unaware of this (lets say SHIELD has someway to prevent telepaths from getting a read on the situation), until someone starts actually analyzing Avenger behavior. And get suspicious. Cue rescue mission.
I'd prefer the X-Men to do the rescuing, but you can throw other teams in if you wish. I just want to see Xavier beating himself up for not noticing.