I'd like to describe one category of abilities that is the least likely to return: tools added in recent expansions that ultimately contributed to homogenization. BC is generally well-regarded historically when in comes to WoW PvP (and raiding). But then we gave everyone 4 new abilities in Wrath; then 3 more on top of that in Cata. And then reworking the talent system in Mists and Warlords actually introduced a ton of new active abilities and procs (since the old talent "trees" actually were mostly passive modifiers with only a handful of active gameplay drivers).
Along the way, we made what I'll freely admit were design mistakes. The underlying one was probably just committing to adding X new abilities each expansion even when many classes genuinely were complete and functional packages without any need for that many new abilities. But then more specifically, coming up with fun and desirable new abilities is hard, and a bit too often we turned to the easy path of looking for a class's weaknesses and offering solutions to those weaknesses. Paladins lack mobility when Freedom isn't up? Let's give them them an array of passive and active sprints to choose among. Shamans don't really have CC? Let's give them Hex, a stun, and a root. And so forth.
Subjectively, removing those things now feels like a nerf, and objectively, it essentially is one. But in the context of broader CC and cooldown disarmament, it's not coming at the expense of balance. And offering new tools, especially through the PvP talent system, that can accentuate strengths, seems like a better path than continually shoring up weaknesses at the expense of diversity.
Finally, I'd like to respond to the idea that this is all about increasing accessibility for new players at the expense of veterans. It is not. The new player experience has always been fairly streamlined, with only a handful of abilities available, and new ones meted out over the course of many levels. We don't think that the existence or absence of a cooldown or CC tool at level 75 has any impact on a new player's ability to approach and enjoy the game. But it does have impact on the clarity of gameplay at max level, spec homogeneity, and how impactful each individual ability can be in a world full of analogues and counters.