If it's a remake, they could easily add in some extra areas. Like say... a Sevii Islands type thing?
Like, take a ship from Slateport City to an island full of the extra Kanto and Johto Pokes and some Sinnoh, another island with some more Kanto and Johto's and mostly Sinnoh Pokes, and then an island further away with many areas housing Unova Pokemon.
Completing quests on these islands would unlock E4 rematches, as a result of how long it would take, Steven has gone into emo mode and hibernated in Meteor Falls with Wallace becoming champ.
Like someone suggested above, Wallace's rematch is he's champion and the 8th gym rematch it's Juan.
Also, on these sidequests, you bump into some of the Gym Leaders and Elite 4 members and that will explain how Drake ends up with Haxorus and Sydney ends up with Krookodile.
Also seeing Unova Pokemon socially mixing with Pokemon we know and love would be amazing. Being in a double battle and seeing Braviary and Blaziken doesn't look right to me...