So I've recently come across the idea of making a Fire Red hack to add Johto into the game, able to be explored after beating the elite four. I'm not a rom hacking expert yet, but I think I could manage figuring it out for such a project.
-A post-game Johto region.
-New areas around the Sinjoh ruins, with Sinnoh Pokemon.
-All Gen1-4 Pokemon obtainable in the game + later evolutions (Sylveon)
-Gen7 battle mechanics
-Some reusable TMs, and consumable(but re obtainable) TRs
-Day/Night, Weather, and Seasons
-Changed "road blocks"
-A slightly changed story
Story changes would include the protagonist of the opposite gender of the player, adding more character development for the rival, etc. I also- if there's enough space left, want to add the Sinjoh ruins, and create a new area around it for catching some Sinnoh Pokemon. Besides, are you going to try to tell me that there's ruins which have a great cultural, historical, perhaps even religious significance, and people aren't turning it into a tourist trap?
After collecting all eight Kanto badges, you can travel to Johto, and take on the Johto league. After collecting all Johto badges, you can challenge the Elite Four again, and they would be stronger the second time around.
Couldn't this be done in gen2? Yes, but it probably wouldn't look as good.
Why not just hack gen4? While making a gen4 hack sounds like a "probably", a gen3 hack remains a "certainly." In other words, while a lot of the things I want to do might be possible in gen4, all of these things are definitely possible in gen3.