You'll never struggle to find groups or players out in the open unless levelling below the current content (<100), still very impressive for a 13 year old game. Good time to come back as the new raid has just launched and many are regarding it as the best one for a while.
I've just relogged after not playing for any length of time since 2011.
On my main as such, I've managed to smash my way to lvl 96 with little effort and have hardly taken in the content. Half the quests disappear. I think I went in 2 zones on Mists before I was in Dreanor.
I haven't let purchased Legion.
Should I race to level cap in Legion? Is there any point running the WoD etc content fully before hand? I expect there's a few who don't yet have Legion capped at 100 so I could get some milage there.
I have this completionist attitude sometimes but then I want to be running raids and making my WoW time worthwhile and more social.