Hm, I think comicbook-universes have the most options available.

They could turn out to have a mutation like Wolverine and Sabertooth.
They could be Highlander-immortals if you want to go with crossovers.
Someone has some kind of serum that is able to bring the dead back.
They aren't really dead but just in some dead-like state?
Captain America's serum seems to be able to keep him from staying dead. Advanced healing maybe?
Then there's the question what the Extremis is able to do with Tony.
Tony could invent something.
Alien technology.
Magic.

I think you have a lot of options :-)