This is a pretty interesting question, because when you think about the basic design philosophy of GW2, where you want to be able to see instantly what spells or skills are being used, you run up into a problem with Mesmer spells from GW1, that Mesmer spells are unspectacular. You don't call down fire from heaven or pull up dead enemies, you stop a spell from finishing or you redirect energy from an attack back on the attacker. Those are hard things to make visual.
I like a lot of the ideas put forward so far. Maybe for interrupts, instead of canceling the spell itself before it emerges, the Mesmer sends the spell back or blocks it with some sort of magical wall. Imagine how much more valued Mesmers would be if you could see an elementalist call down down meteor shower and just before it hits have a shimmering wall of energy deflect it. Or reverse pheonix so it turns back on the caster. Or have illusionary chains wrap around a warrior preventing them from landing an attack. That may be difficult to implement, but could be pretty spectacular.