Give Sentinel a unique "aggro". I mean a table that only the one at the top of the table is actually aggroing the Sentinel, like call it "inescapable". The Sentinel would get extra damage and res/def vs this unique target. Making the Sentinel the perfect "off-tanker" and completely prevent the "target" from fleeing. This would somewhat up the damage on the Sentinel, without turning it into a Blaster, or a death machine like Stalker/Scrapper. They wouldnt steal the tank place as they would really only hold 1 target at a time, but at least it would never flee and would warrant the defense side of the Sentinel. If two Sentinel are targeting the same enemy, only the one with highest aggro is getting targeted, but both get their "DPS/RES/DEF" bonus versus him. (This could actually open new tactics of manipulating a boss into a different position).
Particularly intersting if the Sentinel as high Def/Res versus something that the tank doesnt. (Say an Inv tank with the Psi hole, the Sentinel grab that particular mob and keep it's attention while the groupe defeat everything else. The Sentinel has everything to handle this particular mob, and it will not direct his attack to the tank even with his aggro tactics).
Maybe the easiest way would be to add to the R1 attack a 15 seconds debuff on an enemy that cause extreme aggro (temporarily) and buff the DPS/RES/DEF of the Sentinel vs that target (only that target). If the debuff is applied to a second target, the first one is terminated. Might be easier to set up for the Devs.
Just my 2 inf.