If it's one of the Avengers it can't be Evan's or Hemsworth. They require months of a training regiment to get them sufficiently bulked up to play their parts. That's why they were hunched over in the background during the final Avengers credit scene at the food store because it was a last minute thing and they weren't properly able to prepare so they did their best to hide their lack of muscle.
The show doesn't have the budget for the Ironman suit effects either (it's always almost all CGI aside from the helmet and torso plate. RDJ is only wearing the actual suit in scenes where he's just standing there because it's too clumsy to really walk in well) although RDJ could show up sans the suit.
Renner seems like the most workable though. He doesn't need to get super buff for the part, they can easily do his arrow effects and one of the current lose plot threads in the MCU right now is what happened to Hawkeye do the events of TWS.
The show doesn't have the budget for the Ironman suit effects either (it's always almost all CGI aside from the helmet and torso plate. RDJ is only wearing the actual suit in scenes where he's just standing there because it's too clumsy to really walk in well) although RDJ could show up sans the suit.
Renner seems like the most workable though. He doesn't need to get super buff for the part, they can easily do his arrow effects and one of the current lose plot threads in the MCU right now is what happened to Hawkeye do the events of TWS.