>>51733732first of all make it a Johto remake
around 10 year jump from the original games to align with the current timeline.
I mean, Sinnoh went to the past, lets have Johto go into the future (present)
Postgame Kanto, Sevii Islands, Battle Frontier
The threat is Team Rocket is back (again) with Silver as the new leader. You see how all the gym leaders have changed from both regions. Things like a lot of the older guys have retired and aren't gym leaders or elite four anymore, but you can find them in the post-game.
Since it'd be a 10 year leap it would be harder to make new variants of pokemon but it can be done. Like a new Porygon and other artificially made pokemon could have alternative evolutions. Fossils normally found in other regions that have been newly found in Johto/Kanto so they discover that even fossils had regional variants. Like a Johto Rampardos and Bastiodon that fixes their problems.Or straight up just have them fix the SwSH fossils to their true forms and explain that the archeologists in Kanto aren't idiots. Could even have pokemon newly introduced to the Safari zone unable to grow properly because the different conditions, similar to how Exeggutor grows better in Aloha but is a manlet in Kanto. and its always easy to give legendaries new forms. And a last note, finally give a baby Kangaskhan form and a baby form to connect the Tauros and Milktank lines