Iron Man & Captain America: Into the Multiverse

The worst thing is not, in fact, the giant rift blown right through the space-time continuum. It's not the terrible groaning of their universe as they're sucked through the other side. It's not the fact that Tony is once again hurtling through another wormhole. No, the worst thing is that when it's all over, he's left staring himself right in the face. And the kid is right there next to him, watching the whole thing play out like some horrible soap opera.
Or: Tony and the team get sucked into an alternate universe where Tony is evil and the Avengers are gone. Tony sacrifices himself for the team and gets trapped with his evil doppelgänger, and in the race to save him, Steve comes to terms with some feelings he may or may not have.