i definitely agree with having a Batarian, especially if you were a paragon for ME2 and were nice to all of the Batarians you met. i think it would be an interesting dynamic because Batarians have a much different morality system than most of the other species and there is a lot of conflict between Batarians and Humans. i think that especially if there is some recruiting of species to fight the Reaper invasion a Batarian squad member is a must, need to work with Batarians through some semi-trusted ally.
i really want a Volus, Elcor, and Hanar on the ship. i don't know how well they would work as squad members, but they would be great as crew for the Normandy. They need to be interactive to the extent that Joker, EDI, Kelly, engineers, Dr. Chakwas, ect are, otherwise it would just be window-dressing. Recruiting them for your crew could possibly even bring new upgrades, weapons, and shop bonuses. i'm not sure about having something like a loyalty quest for each of them would be necessary, but it would be interesting. It would also give a chance to learn a lot more about these other species.
Another Quarian would be interesting, especially if it was an outcast.
i never thought about a female Turian, but from Garrus' stories it seems like they are reasonably common in military positions. i think having one would be interesting.
Having a Vorcha would be interesting, but i don't like the idea of having some super smart one. i think it would be much better if the squad member was instead a squad of three vorcha that fought as a unit, with each vorcha being less powerful than your other squad members. They would be really good at being diversions. How to control them would also be an interesting dynamic. They would also be vital for any mission dealing with poisonous gas leak or disease outbreak (though it would need to be pretty original to not seem like another mission like Mordin's recruitment).
i also wonder if they are planning to make Joker a playable character, obviously as some biotic. Need to find a satisfying way of dealing with the Vrolik's syndrome, though. From the sounds of it, acquiring biotic powers through exposure does NOT heal tissue but often damages it (cancer and other complications).
While it may be interesting to find a Prothean in stasis and have them as a squad member, it may a little bit of jumping the shark.