All comments are on random regular BGs (not epics), since they are the core of WoW’s PvP IMHO.
I don’t care for Arenas and would rather see it deleted, so I won’t comment on it.
Solo queue rated BGs mode: no/small gear variance and templates
It works for every other game, but Devs refuse to put it into WoW for some arcane reason.
This alone would entice many players from other games and quite a few who are already WoW veterans.
It would cost money, but have a very aggressive anti-bot squad banning/modding people and bots on sight and make the PvE equivalency for rewards worth it (EG: make a win give 2k gold or something and a loss 25-50% of it). Also Conquest acquisition should be doubled or tripled, and losses should award SOME conquest (10-25%) instead of none at all.
As it is right now, you get 0 gear, 0 conquest and a pittance of honor for losing.
There is pretty much no incentive for sticking to the end on a losing BG other than sheer willpower or drive to win. The 15 min. debuff is even better sometimes since it gives people time to make some real progress on the world (gold etc).
Random, phased World PvP events (think of something like scenarios), with decent rewards (a temporary buff, exclusive access to a dungeon/raid for the winning side etc.) on the world would be a very welcome addition.
New BGs, or new BG skins would make the stale state of BGs a lot more fun.
This goes for PvE too:
Requirements for player power shouldn’t exist in the game: Time/dedication should be enough.
Let the reward for elitism be much faster gearing and unique transmogs and mounts, not item level. Let PvPers fully enjoy PvE gear progression too.
Finally, I’d focus on either 1v1 balance between specs and completely remove dedicated healing from PvP, as all it does is make encounters last longer or frustrate DPS, since one single heal can negate 10s of seconds of DPS.
Healing being so powerful only makes healers the #1 target in BGs.
CCs should all be slightly longer and on a single DR category.
The best state for PvP gearing was WoD. You could actually get BiS without touching rated Arenas.
I can’t say PvP balance was ever great though, but it’s never been too bad for too long - we had at most 1-3 under/overperforming specs.
IIRC the best patches for PvP balance were the last patches from MoP or WotLK.