There really should be a seventh class called the Paladin class (or something like that), and just about all the current Sentinels should be moved there so that we can have a true Sentinel class. Sentinels are supposed to have a combination of biotic and tech powers. In reality, "Sentinel" in MP came to mean "has Tech Armor or another defensive ability." We have 3 Sentinels whose only "tech" power is Tech Armor, 2 other Sentinels whose only "biotic" power is a grenade, and 2 more Sentinels who don't have a single biotic power. I think the Turian, the Batarian, and maybe the Warlord are the only Sentinels that I would consider to be true "Sentinels." Seriously, how much more awesome would the Human Sentinel be if he had Warp/Throw/Overload?
I like the "Medic" class idea too. Make it a truly support-oriented class, with regen powers (maybe even an AOE grenade that recharged shields), crowd control and debuff powers, Neural Shock / Sabotage / Dominate, etc.