How hard is 85 to 100? Or should I level boost instead?

I used to play a druid tank in early Cata (I think I stopped before Firelands?) So, it's been a couple of years, my druid's in feral spec / tanking gear for that level, in an inn in stormwind. I recall running a lot of heroic ZG/ZA and a few starter raids in Cata. I think I had a couple of low level professions, that's it.

Given that, how time consuming is the MoP/ WoD content going from 85 to 100? Is it worth spending the Legion level boost on this druid? I wouldn't even know where to go to start questing, to be honest.

That said, if the recommendation is to "NOT WASTE" the level boost on my 85, what other character would your recommend? I love tanking, but haven't tried any other tanking class. Also, back then, both the melee DPS and tanking spec for a druid was Feral, I hear now that's no longer the case, so dunno what to do honestly. Level as tank? Level as DPS and switch to tank at the end?

Since I don't have any alts, I will essentially be creating a level 1 character and boosting it to 100, if not my existing druid. Any suggestions?

I imagine getting into queues as DPS is gonna be hard, so I am looking at something which will help me get in queues while at the same time being good enough to solo quest level along the way. Another druid ? Warlock/hunter?