actually there is a gen3 romhack with johto...
the problem i see is that gen2/gen4 also included a timeskipped kanto which impedes implementation
a transition idea from my side would be that an npc invents a time machine which malfunctions sending the player to new bark town 2 years in the future (or so ^^)
i wouldn't scrap adv ou cause i'm good at it ^^
srsly though, because our game is transitioning towards gen IV it would add more variety in playstyle
since the physical/special split was one of the greatest impacts on comp play ever in pkmn history
as for the replacement of nu:
it would be more fair to replace it with doubles imo
personally i would rather see that replaced with dpp instead of a third uu tier