I was just thinking about this. Lets assume mesmers come back. Some people like myself think that they might be one of the two other adventuring professions. Maybe pick up a sword, be more rounded and all that. Or they could stay as pure casters if they come back.
So I was just wondering, what if they evolve into more of a jack of all trades class with adding party healing/support. Sort of like bards in D&D. I could kinda see them as illusion/domination magic, swords, guns, and maybe songs/chants from the paragon (who shouldn't be returning).
Another thing to think about: casters have a support char (water el), tanks do as well (guardian), and since the assassin is going to be the other adventuring class, a mesmer-bard would make sense in terms of balance.