I'd add the Sevii Islands, and put the battle Frontier in Sinnoh, as it's the exact same as the one in Platinum. I'd also make another key item, A GB Viewer, which changes the graphics to that of Pokemon Silver/Crystal/Gold. Just my personal opinion...
That would be awesome (the original graphics thing. And I'd like to see Sevii Islands as I've never played FRLG
all the functionality of HG SS
eg: rebattle and trade with gym leaders, the battle frontier, etc...
i would allow players to travel to every region.
the player may change their avatar appearance (hat color, shirt, pants, bag, skin)
and then may chose which region to start in and that will alter the story.
and having to deal with all three teams er...4 (rockets, aqua, magma, and the other lame one...[IMO])
but yeah make some pokemon more night and day exclusive (i hate being able to find starly at night in DPP for example)
but that what i would do and i wouldn't dare make any more pokemon there is enough...
just new regions.
I really see that as a possible Wii game. That would be awesome!