Mostly, I'd like to see a Quarian Soldier. It's my favourite race and least used class, so combining the two would give me reason to play through 1-20 again.
I could also pretend it's because I have this vision of a really fun ability for them, where they use their Omnitool and carried equipment to place down a high-powered Turret that stops them from moving while active, but deals lots of damage to whatever it hits..... But quite frankly I'd just settle for a guy with a Defence Drone and a slow firing missile launcher so that I could pretend to be Kal Reegar.