I just don't see any way that one of the Avengers could show up on the show. I would love it, but the financials just don't make sense. Everyone in the Avengers film commands at least a $1 million price tag, which is far too high for a single episode of a TV show, even if it is owned by Disney. The sole exception is possibly Jeremy Renner, who isn't quite at the level of the others. Yes, Sam Jackson did it, but it's been well-established that enjoys what he does and will pretty much show up in anything if he likes it.