I can't say much without giving away where the source comes from if this gets big, but I had access to part of the script as well as some visuals for development reasons i won't disclose.

Hope you're excited for the return of SECRET BASES! No one seems to have caught the hint in Ultra Sun or Ultra Moon ;)

No armor forms, no new megas, no z-moves.
No "Galarian" variants, England was not ecologically diverse enough for justifying it, but there are "Ancient" variants of fossil Pokémon (dragon/flying Aerodactyl) (dragon/dark Tyrantrum) (bug/water Kabutops), as well as some new ice-age Pokémon (one based on wooly rhino and one based on sabretooth, filling in for fossil pokemon this gen), that are all accessed through a special time machine storyline that starts after you get your 7th badge (optional)
no Kalos postgame but that doesn't mean you don't get to go there. let's just say that time travel accounts for continental drift.

Sorry I don't have a timetable for you, it's been a while since I worked on anything for this, but i'd imagine early-mid May