Literally I just want 2 things. First is for Sevii Islands to either come back, or get reimagined as something like Orange Islands. Those islands are among the biggest reasons I love FRLG so much! They added so many kinds of locations like Volcanoes, Ruins, Ice Caves, Forests, Canyons, Seas, etc. that gave Kanto a world of variety.
I think a redesign could be more like likely since a part of the Sevii Islands focuses on Johto Pokemon (Like the Ruins where you could find Unown, and the Team Rocket Hideout where you battle the Admins from Gen 2) and they would probably change the story about finding the Ruby and Sapphire to interact with other games.
But hey, if they do redesign them from scratch that brings a whole ton of possibilities for new locations and new adventures!
I would even be ok if it was less islands instead of 7, as long as they're huge and as diverse as Sevii were.
Second thing is some kind of Gym Leader Castle/Battle Tree, where we can battle tons of Special trainers like Gym Leaders, Rival, Cameo Trainers, Blue, Red, etc.
Gen 1 never had any sort of Battle Tower equivalent or Gym Leader Rematches, so this is the perfect time to give it one!
I want these games to really have that "I wanna be the Very Best, Like no one ever was" feeling, and theres no better way than letting me duke it out against all those characters.
Most previous gens usually copy/pasted features (Trainer Hill in 3, Battle frontier in 4, Maison in 6,) so it would be nice to see some of the stuff from USUM here. things like Sidequests, Surfing Game, Collectathons like Mega Stones/Zygarde Cells/Totems stickers, and mini games like a Photo Club would be cool ways to flesh it out, while Gen 8 can expand upon the hardcore battle centric ones like Battle Frontier and PWT.
(Like give us Gym Leader Castle in LGPE, and expand it into the PWT next gen since the base will already be established here)