-Leveling and max-level world content is (still) embarassingly easy and needs a serious rework. 7.3.5 did a lot for leveling but far from enough. At 110 you spend 1-2 afternoons gearing up and then you're soloing everything that isn't grouped, instanced content. This does not improve overall enjoyment of the game just because it means you're getting to 110 quicker, and get geared for the latest tier faster.
So why is this an unpopular opinion? Mostly because it goes a lot further than most seem to be willing to think about. Mindnumbingly easy leveling and endgame world content turns all of it into a bucket-list you just want over with. And a major part you want it over with is because it is mindnumbingly easy. Yet whenever you present the idea that maybe part of the reason people are so bored outside of their weekly raids is because all non-raid content is designed to be guaranteed success, the only thing you hear back is either 'I've done it before' or 'Why should they slow us down?', as if putting everything on ultra Ludicrous Speed is beneficial to long-term enjoyment. It isn't.
-WoW: Classic will eventually need some rebalancing. It'll probably take a while as both Blizzard and players are reluctant to accept it, but leaving a stale, unchanging shadow of 1.12, leaving OP classes OP and useless specs useless won't work for more than a few months, at most.
Eventually we'll start to see some balance changes. No major ones, but some. If only to prevent 80% of all characters to be Warr/Priest/Rogue/Mage.