Whether 5e is big enough for it, I don't know. Whether WotC will do it, I strongly doubt.
But what I want for 5e more than anything is a WotC book entitled Advanced Dungeon Master's Guide.
It would be nice to have some expanded combat options in such a book—the new "weapon actions" added to Baldur's Gate III are a promising start, and may indeed be intended for eventual addition to the tabletop game—but that's not the main thing I would want.
What I want has two components:
1) additional, optional rules for a whole bunch of specific cases that come up relatively infrequently yet often enough to make an official rule desirable for tables that want that—along the lines of what we have already been given in a highly piecemeal and incomplete form when, for example, the "rules expansion" books give us rules for falling onto water, sleeping in armor, and so on. There are a whole lot of things my players do pretty often that really ought to have some rules, in my opinion. Obvious example: one PC wants to use their action to toss an item to another PC, but that PC is some distance away. Can they successfully do so? 5e offers no guidance.
2) a substantial section on how to set DCs, with many more examples than the DMG provides. Setting DCs for things that players want to do is one of the most frequent tasks that 5e demands of Dungeon Masters, and one of my least favorite things to do when running a game. No matter how much DMing experience I have, it always feels like I'm pulling numbers out of thin air—and that's because the official guidance in the 5e DMG on how to set DCs is essentially to do just that.
Every time I express a desire for a more robust ruleset, folks pop up proclaiming that what I want is contrary to the design philosophy of 5e: "rulings, not rules." And that's probably true. But I'm not looking for a crunchy system like 3.5e, just a more robust version of what 5e already provides. Many DMs feel exactly the opposite, of course, but many others also agree with me that it's more difficult and less enjoyable to constantly come up with new rulings out of nowhere than it is to master a set of preexisting rules for the same situations. And given how 5e presents all additional rules as optional rules, I really don't see the difference between "more rules, fewer rulings" and "more guidance on how you might make different types of rulings."
There are enough of us out there who do want this, that we've made (for example) Level Up a successful commodity. Still, Level Up won't work for me because it changes the rules of 5e, and that won't fly with my group; switching to an entirely different system is even more inconceivable. But that's not what I want anyway; what I want is a product similar to Level Up that limits itself to adding to the 5e ruleset in order to shift at least some of the responsibility off the 5e DM, who, as it stands, must constantly make freewheeling rulings with little to no official guidance to back them up. Official would be preferable; third-party would be fine—but I've never even seen a decent third-party version of what I'm seeking.