Good question. For me they would be:
The Black Widow. Marvel needs to push more women into prominence and Warren Ellis proved in his Secret Avengers run how intelligent she can be.
Spider Girl. Bring back May Day Parker! Have her travel back in time or something, wind up in the 616 MU. Have her team up with Anya Corazon and call the title Spider-Girls!
Jimmy Woo. I loved Agents of Atlas and I would love to see those characters return in some capacity. Jimmy Woo as the leader of one of the world's most powerful secret organizations should be a prominent member of the MU.
Those are the only three I can think of off the top of my head.
Did you catch the cameo of May Day Parker in Age of Ultron? It really sucks that it was SpOck who got the flash of seeing May and American Dream in the MC2-verse. Without the real Peter being around, I really care even less about seeing May.