When I think about compatibility, what I expect, and what I want, this is where my head is:
I expect that monsters, magic items, and spells from other 5e sources should work fine with 2024 D&D. Maybe something takes a little tweaking but I bet most of them would work as-is. I'm not expecting that I'd take 2014 D&D spells and mix them into 2024 D&D – there's little reason to bother. It's books like Deep Magic 1 & 2 or other 5e published sources of spells that I'd want to still be able to use and I expect I could.
Same with monsters. They might feel a little different but that's true now already. Monsters from the various Tomes of Beasts feel different from one another and always felt different from the 2014 D&D monsters. Same with A5e Monstrous Menagerie monsters and Flee Mortals monsters. I bet they can still all work as is without any modification.
I expect magic items won't be a problem either. A few tie closely to character options which might take some tweaking but Vault of Magic items from Kobold Press should still work.
Backgrounds probably require some modification by adding ASIs or feats or whatever. I think it'll be easy to add some blanket "use the +2 / +1 or +1 +1 +1 ability modifiers to backgrounds" and access to level 1 feats.
I think races and subraces from older material might work when you remove the ability score connection. Some might be weaker and some stronger but overall I bet they work fine.
I expect subclasses might take a little more work but many of them should probably work fine once you move the 1st level feature to 3rd. Some of them that expect baseline character abilities – like paladins with smites – might not work perfectly. That's true also when trying to take a 5e subclass and applying it to a Level Up Advanced 5e character or a Tales of the Valiant character. You can do it but it might take tweaking and conversations between DMs and players to do. That's fine.
Adventures and campaign settings, of course, should be mostly fine. The DC ladder is the same I expect. There's little mechanics in those books that focus on particular class abilities that might change. We've been converting adventures from entire other systems without much effort so I don't think that's an issue.
It makes perfect sense that a group will likely want to select a core ruleset when they choose to play a 5e campaign, and everything else surrounds that core ruleset. That said, that core ruleset can likewise be modified. I'm running Level Up Advanced 5e right now but it's easy to replace inspiration with the Tales of the Valiant Luck system, which I prefer.
I can also use monsters from anything in my A5e game. We didn't include other subclasses but I bet we could use races from other systems if we wanted to, but A5e has a ton already so we probably don't need more.
I guess we'll see what the practical implications of this all are when the books come out and we actually start playing it. I don't know that we'll know what we're really going to do until the books hit the table.
