There are plenty of comps viable.
And when you look at tanks and healers there's a lot of combinations there.
Healing wise resto druid is looking quite good for mythic+; so are Shaman, Paladin and Holy Priest. Disc priest not so much, but that's to be expected. I'm not sure about monks as I haven't checked into those much yet.
Tanking wise there are also several options. Some of the tanks might be stronger for a specific version of mythic+ but there won't be huge discrepancy and most can do their job I'd say.
DPS on the other hand seems to be a bit different. There are quite a few number of specs that simply do not have the toolkit required to be good enough as certain things, such as AoE dps.
Of course that doesn't take away that you can still easily perform with those specs and if the group is somewhat organized they can even work around it to some extend. But for a pug mythic+ or a situation where you get to pick and you have equal choice; those things play quite a big role.
For druid specific. Feral seems to be so-so on a lot of stuff due to their lack of proper AoE damage. It's very situation dependent though. If the AoE mobs are infrequent and die fast there might be a role for brutal slash. If most AoE consists of 2-4 mobs with quite a bit of health there is a good chance that multidot AoE works well too.
Balance might be a bit better off overal. A lot of people seem to complain a bit about resource rampup, however with proper resource management and pooling before the next pull, you can probably do good AoE dps by starting fights with (near) full AP. In a mythic+ dungeon you'll mostly be chain pulling. And you still have moonmoon and maybe even Astral Communion.
I think it will be good if we have at least 1 dps spec viable, preferably both.
Having them needed is not as big an issue to be frank; I'd still rather take a good viable-yet-not-needed class over a bad or so-so "needed"-class.