Lots of great choices already listed. Shout-out to the poster who suggested Terror!
I'm trying to think of who, if included on the Avengers, make it a must-buy for me without knowing the creative team or any other contextual info; who hasn't already been on the team but seems like a reasonable addition (sorry, Slingers and Phil Urich); AND who hasn't already been mentioned. I'm also going to pass on any Spider-clones because that's too easy an answer.
My choices:
1. Crystar
2. Josiah X
3. Gali, Daughter of Galactus
I'd read the heck out of a team that featured any or all of these great characters.
-Pav, who loves a weird Z-Lister...