Now that homogenization is being reverted there's tons of new niches new classes could fill. I don't think new classes are infeasible for Blizzard to balance anymore.
And turning more classes into hybrids doesn't sound fun to me either.
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No it wouldn't. New classes with new roles doesn't change queues at all. Lazy DPSers will always be lazy DPSers. We just got the 3rd new tank class in a row and there's STILL a tank shortage. All a new pure DPS class would do is make a lazy hunter reroll into a lazy whatever new class it is.