Kal Reegar - Yeah yeah the email SAYS he died. But maybe he didn't. Pay Adam Baldwin what he wants. We want Kal Reegar. Shepard receives email from Kal'Reegar and you actually get to land on Palaven. Rest of his squad is wiped out once more. You successfully help him get extracted from Palaven and then aboard the Normandy he offers to join you but if you decline he'll become a war asset under Quarian.
Balak- Lotta fans would hate this one but where fans have hate I see potential for a good emotionally engaging experience. Balak kills craploads of humans. Shepard killed a crapload of batarians. Imagine the tension and things of that nature between them and how over the course of the game that connection weakens or strengthens. I'm thinking I have to go with Terra Nova on this one. Batarian/Human conflict still big deal and Shepard personally goes to resolve it etc and as a gesture of stubborn gratitude Balak can either join Shepard or go back to the Batarian Fleet. Citadel:Batarian Codes must be completed to trigger.
Turian Female of some kind- Drell and Batarian females can slide because they are not active combatants, but civilians in the war. But Turian females are CLEARLY combatants. A squadmate is a perfect way to introduce female turians in, reminiscent of how it felt to have Tali, the only quarian in a game back in the original Mass Effect. This is extra but if fans want a drell female to be seen badly, then ideally the mission where you recruit the turian female should take place on kahje.
Randall Ezno- a good opportunity to take advantage and revisit the second planned DLC pack for the original Mass Effect that never came to be. That is, the Cerberus facility on planet Misery where experiments are performed on humans. Make it the advanced cerberus troop factory and have it run by the director. Go help Randall kill the director. Get a squadmate.
I would like a dirty dozen but as you can see, I only managed to jack it up to 11. Interested in hearing others' ideas