Stealth mechanics wouldn't be impossible to implement. It wouldn't work like a typical stealth game, like Splinter Cell, though. It can't, with the action oriented squad based combat system.
However, I think a system involving backstabs would work. Like, if you get behind an enemy and heavy melee, you should get a special kill animation. Different classes could make this work in different ways. Infiltrators can obviously stealth behind enemies and nab them. Engineers could use their drones to draw attention allowing you to get behind enemies and backstab them. Vanguards might take an upgrade that allows them to charge directly behind an enemy instead of in front of them. Soldiers can obviously use bullet time to maneuver around enemies and get behind them. Sentinels can stun with tech armor allowing them to move in for the backstab. Adepts can use their singularities to hold enemies in place while they move for the backstab.
It would reward players for playing smart and would open up the chance for some very rewarding kills. And each class could have their own unique kill animation, right?
Soldiers can backstab with the omni-blade.
Infiltrators get to neck snap.
Vanguards get a biotic neck snap.
Adepts use both hands and biotics to cave in the enemy's head (think Red Viper vs. The Mountain).
Engineers might plant a bomb on their heads then kick them away and let it explode.
And Sentinels could decapitate with both ombi-blades.
Again, it wouldn't be steal per se, but it would be a cool mechanic and for Infiltrators, it WOULD really play to the idea of being a stealthy assassin.