If they don't explore Widow/Hawkeye in a Shield film (or Captain America 2) then I suppose I'd rather see both of them return for the Avengers 2 rather than introducing Ant-Man/Wasp, since there needs to be a lot of exposition to explain the latter couple.
Without Ant-Man/Wasp, there is no Ultron. Without Ultron, there is no Vision. And, I need my Vision as an Avenger.
If no Vision, then perhaps Black Panther will do for a male addition. Plus, it may be good to add more diversity.
One thing the sequel needs is a new female addition that can go toe-to-toe with the Lokis through physical force. Ms. Marvel and She-Hulk would be, if Wasp isn't added. Perhaps Valkyrie, if she is introduced in Thor 2.