I really like the new areas of Unova B2W2 added to the Region. It doesn't feel complete without them, plus it gives the option of New Game+ for Unova.
It could also include: PWT, Challenge Mode, Pokéstar Studios, Medals and Join Avenue.
Obviously this is going to take a while to add, but it also means Johto won't be the "final" thing added to the game, since Kalos and Alola are unlikely.