Soldiers should have more active powers. Switching between modes of fire, grenades, maybe special melee/movement skills to reflect their peak physical conditioning.
Engineers should get more direct modes of attack. Sitting in cover using drone then spamming Overload is only fun for so long. A special drone that is made, not to distract, but to deal lots of damage would be nice, or a killer turret akin to the Cerberus engineer's. A skill that jams enemy weapons in an area. Maybe some sort of grenade launcher power that makes them more effective when using them?
Sentinels should be the only class that can both prime and detonate combos. Adepts would use raw biotic power, while Sentinels focus on versatility and combos on top of their powerful defenses.
Infiltrators and Vanguards seem perfect as they are.