I'd think I'd see Sunshine on the sequel to the DS and Galaxy on the DS' sequels' sequel, in keeping with Nintendo's current pattern:
GB: DK94, remake of Donkey Kong and first Mario arcade game
GBC: SMBDX, remake of Super Mario Bros and first original Mario NES game
GBA: SMA2, remake of Super Mario World and first Mario SNES game
NDS: SM64DS, remake of Super Mario 64 and first Mario N64 game
Therefore, logic follows that Sunshine's sequel would be on the DS' sequel, and that Galaxy would follow, assuming the pattern continues. To be truly honest, I can't see Galaxy coming out on a portable console for many more years to come.