As Bungie is shying away from the term 'MMO' they will probably do away with traditional MMO Classes. I'd imagine that the Titan would receive abilities to mitigate damage at the cost of maybe mobility or something to that effect. But to actually attract the incoming fire from PC's is something completely different to taking hits from NPC's. It'll be interesting to see how they make Titans a more attractive target for PC's without getting heavy into RPG elements (which I'm not opposed to). :P