-Remove warforging/titanforging;
Expands power gap from start to end of expansion, worsens stat creep, breaks balance, encourages unhealthy playstyles. Rare drop cosmetic rewards like transmogs and mounts can keep people playing old content, sticking lotto-rare power drops in it isn't a good replacement.
-Remove all unnecessary time-gates;
Everything should be "grindable". Except on highly competitive content like mythic raids/rated pvp, so as to not encourage unhealthy playstyles. Having artificial caps leaves players feeling obliged to do content they might not otherwise want to do at any given time. Or feel "left behind". WoW is a buffet, we don't want to be spoonfed a small portion of everything.
-Make reputation faction wide on account.
-Make BoP gear BoA;
The last two changes will make playing alts infinitely more attractive, opening up a lot of race/faction based content to more players. It adds more replay-ability to the game. Drops across the game gain more value as you consider more characters/specs when evaluating a piece of loot. Less likely to find yourself grinding 99% vendor trash for the 1% upgrade. Ultimately if you did the content, why shouldn't you reap the rewards? If you want to gear up 5 characters, its still going to take 5x the amount of gear drops. This just means more people are willing to do it as they can play the class/spec they enjoy the most at any given time.
With people gearing more alts, they take longer to reach the point where they feel their gear has stagnated, keeping people ultimately playing longer.
-Legendaries in their current form, do not happen again in the next expansion. Power =/= optional in the eyes of players, you can add rare and exciting content to the game in better ways. They can be flavorful utility items, pets, mounts, transmog pieces. Things people will still get excited over, do people even get excited for legendaries these days? Its been more "finally, that RNG garbage is done with" kind of feeling.
-USE THE DAMN PTR;
Incentivize it if needed, blizzard store discounts, recycle cosmetic rewards, get people on there testing content before it hits live and causes a ton of problems.
-(Least likely, probably doesn't matter at this point) Double down on paid services, cosmetics, etc. And remove the subscription fee.
I think WoW has more than enough whales to keep it afloat at this point. If they could get millions of players paying to play, imagine how many more would if it were single purchase. And how many of those would spend more than $15 a month on relatively low effort cosmetic/convenience premiums. Especially if there were a secondary market like steam with knives/unusuals.