Well it's been stated by Shogo that the Masterpiece line homages the G1 Transformers and looks like they still have a lot of G1 Decepticons to do yet.
G2 Sideswipe was just a simple repaint, so no huge investment on Takaras part. So it possible that many of the G2 characters can just be repaints/retools of existing molds if Takara really wants to go in that direction.
But if there is any G2 character and new mold that has a good chance of making an appearance in the Masterpiece line, its G2 Megatron, as much as I want him to be a gun.
I wouldn't be surprised if G2 Grimlock comes either. that's just a simple repaint.
But let us old guys get our G1 Masterpieces first