Alright. Below are the dual types not existing in canon games. Given that I know somewhat the members of the se7en team, you can rest assured they've included some of those in the pokedex. It's also meant to be an inspiration, since the pokedex is not filled yet. At this point I'd like to point out that those are not the only ones to focus on. Most of the already existing dual types are there because it's logical for them to exist, and as such, if an idea of a pokemon with an already existing dual type emerges, don't instantly throw it into the bin. Anyway, here's the area to innovate in regards to typing. Vive la Gratteras