Joss Whedon has said that the Wasp was in his first draft of the Avengers script, and she's been seen in early concept art. I don't know which Wasp, but there's not much doubt that the delays on the Ant-Man movie changed Feige's plan, since if that movie had come out earlier, it would have set up Ant-Man and the Wasp as Avengers characters.

Since Whedon seemed to be more or less following the comic lineup, maybe he would have then introduced Hawkeye along with Scarlet Witch and Quicksilver in the sequel.