Okay they aren't going to whole sale ditch class, race, background, feats from the 5.5e PHB, it's most likely going to be refinements out what they have learned and worked well so far. They will look to the stuff they have done that is popular.
We have a general idea of what races races will be like, I think Lineages rules will be saved for the DMG. They will want to offer more then the regular 5e PHB, so expect some additional races, like Goblins, Aasimar, Genasi, and Orcs could be in. We will need to wait for MP: MotM to get a better idea for how subraces will be handled.
Classes will have Tasha style alternate class features, those were extremely popular. I also think the Artificer and maybe the Psion could make it in. Really, really outside chance of something like Blood Hunter or Warlord making it in.
There maybe some spells from outside the 5e PHB that get in, and boring unpopular spells will get cut. I expect more conjugation spells like Summon Celestial, Summon Dragon, and Summon Aberration then the previous conjure spells. Even Planar Ally is not well designed.
More suggestions inspired by Strixhaven on the social and exploration pillars, but not at the expense of combat.
The Patron system could make it in.
There could even be a more generic piety and faction systems in the PHB if they can find the room for it. Piety system could be based around Domains instead of individual Gods, replacing Domains as the Cleric subclass with something broader and more interesting.
I think Background kind of suck right now, the feature is just too hard to use in published adventures for example, BUT the space sacrificed by races maybe give backgrounds room to do more.
I could see a Gifts system added, like Supernatural Gifts from Theros.
The biggest change I see is references to Magic the Gathering settings in the core books.
By 2024 I think the merging of the multiverses will be official, way to late to bury it anymore.
Look to MP: MotM to know what the 5.5e MM will look like, it's basically the template, the MM will be that, but much bigger.
The 5.5e DMG will be more focused on social and exploration pillar advise and mechanics, if there even is a 5.5e DMG.
I also think these books will reference 5e books.
Multiverse will be a much bigger focus with less FR as default, which ironically will be good for FR as they will be able to release a 5.5e Campaign Setting Book for FR without fear it will derail their yearly adventures.
And I expect at least one kind of surprise UA I don't see coming.
So more refinements, not a sea change.