I'd silo the progression of each type of gameplay content so that it is completely seperate from each other. This means removing leveling, but keeping the campaign. There is absolutely zero argument for forcing people to level a character to max level just so they can play a co-op game with their friends. Character power comes from progressing in each silo of content and is specific to that content (M+, Raids, Delves, World content).
I should be able to play whatever class or spec I want, and I should be able to get a friend to play with me without them being forced to spend hours and hours getting a character to max level they might not even enjoy playing.
MMO's are a dated concept and carry way too much baggage which takes away from what is otherwise a really fun co-op game.

I'd also make it so you can see players are NPC's more than 100 yards away

I'd also completely remove the concept of 'realms' and just use the already existing phasing shit to keep all areas populated (As it was intended)