a little thing I've been thinking about is cat type. Strong to bug and flying (cats eat bugs and birds) weak to water (cats don't like water) weak to electric and steel (ever seen those cat vs toaster videos?) strong to dragon (because right now dragon types are overpowered in my opinion)
I had a dream last night where there was a light type which had moves that had different effectiveness based on the color of the opposing Pokemon. That would probably be stupid, though, and definitely over-complicated.
I don't think any new types are a good idea, personally. Anything "cosmic" I guess is most viable to get its own type, but I have difficulty seeing how its type matchups would be.
We don't need a light type, I think, because we have psychic and fighting. We don't need magic as we have psychic and all of the straight elemental typings, and for "digital", we already have electric.
Most of the others seem too specific and/or non-elemental to me. Of course, dragons aren't really an element, and I'm mostly just biased due to growing up with the same ones for so many years.
Light type only makes sense due to the idea of light vs dark. I'm still hoping sylveon is a light type. Perhaps I'd also like to see sound type as well. There are a handful of pokemon that could fit this category (Jigglypuff, Whismur).
Maybe an air type. Not really flying, as the effectiveness would be different.
S.effective: Fire, Flying
Weak against: Ice