Let's see...
First, I'd drop Liara. Whether you want to characterize her as a Shadow Broker or some Prothean scientist, she's not a soldier and shouldn't be treated like one. You can have her for Mars, but after that, she leaves the ship to go with Hackett and work on the Prothean device. We get Prothean device and asari intel from her in the comm room. You could leave meeting her on the Citadel as it was. That way, those who don't like her can ignore her.
Second, drop Javik. I'll probably get flak from this for booting him over EDI, and while I don't like the sexbot aspect, she at least has appreciable character development. Since he's DLC to begin with, the game runs without him, and I really don't think he contributes anything more than a few one-liners.
So...who to add?
Kirrahe, firstly. I've actually created a fanscript of him added as a character, fully integrated into the Normandy. Here it is, if you want to take a look:
https://docs.google....KQugy28mqJOfj4k
The reason? Because he's a soldier, he'd have a good rapport with the ME1 crew, and given salarians got the shaft because their one spotlight in the story was someone genocidally insane. I blame the writers for the ridiculous glorification of Wrex, but there needs to be an awesome salarian.
Second, I'd add Balak. If the salarians were shafted, the batarians at least never had Mordin. Plus, the tension between him and Shepard would be fun to watch
Third would be one of the asari from Thessia, either the one with the sniper rifle, Lieutenant Kurin, or the one at Outpost Tykis. Those three actually made me care slightly about Thessia, rather than listen to Liara's slightly racist whining. Plus, we probably would need one asari, since they are a Council race.
Can't think of another one to add.