Well, first off I would make sure the trades were all equivalent in quality and difficulty. There needs to be a coordinator that oversees them all, and someone in each that makes sure risk/time vs reward is met. Trades should in theory be able to make very quality content all the way to raid level, not make meh stuff you can kind of use to give your alts a hand up for a few days.
Risk vs reward overall needs to be much more consistent and streamlined. There’s too much stuff in the game that is horrendously difficult or time consuming to accomplish with almost no payoff. Or stuff that’s pretty easy and has a great payoff.
And make sure there’s no gap in loot. Every expansion there’s one or two pieces of gear that are super hard to get. No more of that crap. Make sure your rewards are useful to ALL CLASSES AND BUILDS rather than constantly making them for specific ones (how many months went by before there was a dagger weapon reward in WQ the last expansion and this one?? Why is almost every hunter weapon reward for survival??)
I’d also do the idea I posted earlier about letting you use alts as follower NPCs.
Also, I would tie in old content to new: let you use the farm from Pandaria and the Garrison from Legion for your characters to get some value out of instead of simply abandoning the concept entirely. I would make a real effort to make old content still matter and be useful instead of almost totally abandoning it when a new expansion comes out.
I would try to come up with content and storylines for each class, and each race, so you have more than just the stock storyline to follow.
And when the final end game happened, the PLAYER CHARACTERS would be the key forces that save the day, not the usual stock of pet NPCs that Blizzard loves so much. Enough of having Thrall or some other NPC deal with the enemy, this is an RPG and the players are supposed to be the heroes.
Oh yeah, and I’d fix Gilneas so its usable as a player city.
One more idea: I’d make dungeons scale to the size of the party (down to 3 at least) so you can hit content with smaller groups. I’d make dungeons have special currency that you can then use to buy specific targeted content rather than completely being at the whim of the RNG.
As for the theme, I would do an underground world, beneath the continents: its all interconnected with gigantic caves, etc. What lurks down there? Who lives there? What monsters might be revealed?