I want the series to go on, definitely. I'd love to be a Quarian on his or her pilgramage, with possible tie-ins to a Geth-Quarian conflict. It would be a break from the epic defeat the ancient unstoppable evil, but would allow for more poignant and creative storytelling. That's #1 on my want list for the series.
Alternatively, we go back to when the Geth became self-aware (though that would mean no pilgramage story). This could let us see the Quarians in a completely different light: accepted by the council races, without the need for enviromental suits, with a home-planet and with Geth servants (at least at the beginning of the game).
For other races...
I'd love to be a Turian fleet commander. Decision-making would take center stage in this game, but I don't want an RTS. I don't know how they'd pull this off, but I'd commend them for trying even if it doesn't turn out great.