As time goes on my wish-list gradually increases.
I'm reasonably happy with most of the PHB as-is. I think there are a lot of fairly minor tweaks that it could benefit from, but they are minor - little trims that would just make the game that bit better. My major wish here is that whatever they do to address problematic material in the books they get done. That may be tricky, as I suspect it's a bit of a moving target, but I really don't want to see drow being revised in 2024, then again in 2025, 2026, and so on.
Likewise, I'm reasonably happy with the MM as-is. The major thing here is sorting out the spellcasting trait (and similar) - ideally, everything you need to run a monster would be right there with the monster statblock. The only other things I would do would be to move the tables from Appendix B of the DMG (monster lists) and also the Monster Features table from p280 (creating a monster) into the MM. (Indeed, every 'monster' book should really contain a version of that table for the monsters it includes, to empower DMs who are creating their own monsters.)
But it's the DMG where I would really like to see changes. Although I rated (and, actually, still rate) the 5e version as the best of the DMGs across the editions, I've been shocked at how little use I've made of it over the years - treasure tables and magic items, creating a monster, and that's it. The rest of it is very superficial - it covers a lot of ground but is surprisingly short on detail.
So I'd like that book considerably altered - at the very list there should be many many more pages of traps. But ideally I'd also like to see a much more in-depth discussion of types of adventures (treasure hunt, mystery, heist, extended pursuit...) with a breakdown of each; node maps for short, medium, and long adventures of various types, 1-2 page spreads on a variety of environments (desert, mountain, forest) so that each has a couple of big, noticeable features that means that all 'desert' adventures have some similarities and also feel different from 'mountain' adventures. And so on.
What I expect: actually, I'm inclined to think I'll more or less get my wishes on the PHB and MM, in that I expect fairly minor tweaks. I'm not expecting much from the new DMG - potentially the same book with new artwork. I certainly don't expect them to include anything from Xanathar's or Tasha's to be in the new core, nor do I expect the new core to do anything that breaks compatibility with those books - the Gift Set appears to be a sign of things to come, so I'd expect us still to be using it.
And I'd be okay with that - my absolute minimum is much better binding and a much better index, and surely they must be able to manage at least that?