Having a changing roster is one of the hallmark of The Avengers. So, knowing what you know now, who would you like to see join and who should play them?
I know it's a bit crazy but I would love to see Ms Marvel played by Kate Winslet. Why not get the best I think? She would be great alongside RDJ's Iron Man.
I don't want to overwhelm the roster though so I would probably keep it to adding just one character personally.
Even though I'm dying to see Wanda Maximoff join the Avengers. Claire Foy (Little Dorrit, Upstairs Downstairs) might be cool for that.