I don't want to be a stickler for saying this but I think the actors playing the avengers now are the right ones.
I know I might be getting some bad feedback from this latter, but here me out. The actors they have playing the avengers now are making the character even more iconic, not that hey need to get even more iconic. It's just, when you say Robert Downey jr people will associate him with ironman or if you say ironman people will associate him with Tony Stark / Robert Downey jr.
Also the actors that play the avengers now do a really good job at portraying their character. Personally I can't even put in to words how well they acted as their character, they portray fear, excitement, sadness/loss, anger, willpower and strength. That and Robert Downey jr is the best and one of the biggest smart asses I know.
This is why I love the avengers the way they are.
Ok I know I said re-casting the Avengers in the tittle so here is my re-cast, Jason Momoa as Thor and that's the one I can think of because he is pretty much Thor material. He's big, he has long hair and a beard all you need to do is dye it blond or at least lighten it. He also looks like he could portray a good Thor.
There that's my re-cast and why I don't want to re-cast them.