It's difficult to say for me.
It's a problem which simply arose because the game is 12 years old. Blizzard probably never expected the game to become so popular. So each year Blizzard created new content (i.e. zones, classes etc.) for players who loved the game dearly and continuously asked for more and more content. So it's a natural accumulative effect.
I don't see how they'd be able to re-use the old world zones besides scattering World Quests/Class Quests around Azeroth. It's a shame something doesn't happen (like an Old Gods Cataclysm-esque expansion which changes world, again?) and we don't just start all over again, but this wouldn't make sense as we, as players, are always becoming stronger through each expansion... People wouldn't want to feel "weak" again.
In terms of the classes, I think for lore-related reasons (although Blizzard have created new lore out of thin air time and time again), something like Warlock healers really wouldn't make sense, plus Warlocks are too power hungry for that matter, haha. Plus, I think hybridisation takes away from the uniqueness of each class. If each class can eventually tank or heal, you feel less proud/unique about your Protection Warrior/Paladin tank, or your Resto Shaman/Druid or Holy Priest. They're my thoughts at least. I think if anything, there's too many hybrids nowadays - and the players aren't evidently happier because of it.
Subraces are cool. Would be interested to see how they execute this. The core of Warcraft for me though will always be Humans vs. Orcs or ultimately Alliance vs. Horde.