Well, I'm a bit disapointed. While I expected a 3rd gen4 game, they shouldn't have used up a title that'd have been absolutely perfect for a G/S remake. This may actually be the first time I get the third release in a gen, too, since I don't own either Diamond or Pearl (though that hasn't stopped me from playing Pearl. Depends on if I feel like playing essentially the same game again in a year or not, I suppose.
As far as where else they can go... well, the options are numerous to say the least. They haven't even used up all of the potential gemstones yet. It'd break the usual color scheme, but a Topaz/Amethyst pairing is possible. Let alone what they could do if they thought of something else.