Even if they never make remakes or sequels of Colosseum, they should at least revisit Orre for a mainline game. It already has a gimmick (shadow Pokémon), and they can go back to an actually evil team that involves those shadow Pokémon. The colosseum games had plenty of good ideas with mediocre execution, that could easily be expanded upon or improved. Plus the original games are difficult to access legally and who knows if they’ll ever get ported.