Introduce 10man hard mode again but: in its own unique instances; that way the numbers would be balanced a lot; not everyone will be shoved into the tiny 5man gametype and the only alternative wouldn't be the hard-to-find-people-for 20man.
You can't flex it or re-use 20man or 5man instances for it, because we'd have the same problem we had when 10man and 25man were coexisting, constant flamewars in this forum too about what difficulty is the hardest or the coolest.
As a bonus advantage, the strict metas of a handful of combinations of classes/specs of 5man would open up to more complex combinations of 10man, which doesn't solve it entirely (even 20man has metas) but it's an alleviation.