The biggest change I would make would be to leveling -> Currently PvE leveling is a great way to spend many hours not learning how to play the game, beyond the absolute basics (i.e. how to get around the world at large, etc).
PvP leveling is great! You are slowly introduced to your skills, and are getting experience and levels in the same style of content as you will be doing at max level!
PvE leveling is mindless and boring. You can level just as well with 1 monitor Netflix, 1 monitor porn, 1 monitor WoW. Dick in hand, thinking about if Angel and Buffy will ever bang, mindlessly mashing Frostbolt.
PvE at level 100 is supposed to be extremely mechanic driven, with a high skill cap, where each decision you make second to second can determine if you live or die - standing in the poop, mishandling boss mechanics, or misplaying your class skills.
PvE leveling does not train you for PvE max level, so why are we doing it for 20+ hours?
If I were to rework leveling I would make it shorter, but more intense - you shouldn't be doing content that doesn't prepare you for more, harder content. Level 1-10 should be basic mechanics - movement, basic CC on you and on targets, counterspelling, etc. Levels 11-20 should step it up, giving low DPS requirements, needing to actually deal with counterspells every so often, buffs and debuffs, etc. and so on until level 90-99, which requires using boss mechanics, not standing in the poop, relatively strict DPS requirements, etc. so that people are prepared to not completely suck at max level. It shouldn't necessarily require you to be mythic level ready, but you should at least be able to carry yourself through normal raids without it being too much of a strain.
AKA people shouldn't be able to even reach max level and raid unless they can handle passing the fucking eye to people on Iskar.
I would also redesign what is shown on the UI to accommodate