I know that people have said Mystara is too big to be properly rebooted in a single book, but I think that doesn't take into consideration that a lot of the information can be condensed or would be considered unnecessary for a 5e book.
I have my Explorer's Guide to Wildemont right in front of me. Opening to a random section, it gives an overview of a place called the Berleben. Now, I've never listened to/watched Critical Role and know nothing about it--to the point I don't even know if it's Youtube videos, podcasts, or something else entirely--so I have no idea how important Berleben is to the storyline (I like reading settings and it had archetypes and monsters and stuff). However, they're described as follows: Population, Government, Defenses, Commerce, and Organizations, which are each given one sentence. Then three paragraphs on a description, then a paragraph each for Government, Crime, Geography, and Adventures. I see a few sections also have a paragraph for important NPCs, although this one doesn't. The whole section is just about a page long.
Now, I know that Wildemont doesn't have as much stuff in it as Mystara did, but I can easily see them reducing each part of Mystara to an overview that's one to two pages long.
Now, even if they never touched Mystara, I would like Immortal rules instead of the more typical Epic rules. Or some combination of them. I think becoming a literal godling is a lot more interesting then just becoming super-high level and fighting godlings. But I also think that Immortal/Epic rules would likely end up as their own book rather than an appendix in a setting book.
As for monsters, there are a lot of them that really are just regular monsters but with a different name. Sure, Savage Coast had all those weird hybrids, but a lot of them feel like they could be made with a template. Take a beast, add one of these abilities to it: part frog, gets Standing Leap; part cat, is proficient in Stealth, gets Pounce, etc. A couple of Mystaran creatures were published in the Tortle Package, and Magen are in Icewind Dale. My personal list for being officially converted is: acteons, piranha bird swarms, brain collectors/neh-thalggu, Mystaran drakes (but as a single creature rather than four sub-types), dusanu/rot fiends, at least some of the Mystaran elementals, athachs, gray philosophers, mujina, nucklevee, aranea, thoul (for nostalgia's sake), wyrds, maybe a few others. Obviously, this is just my opinion, but a lot of the creatures I didn't name are, as I said, regular monsters that kinda look different. And, of course, some of them, specifically gyerians, phanatons, rakasta, hutaakan, and lupin, could easily become PC races.