One thing I've noticed quite frequently is that new type concepts tend to revolve around things such as senses (sight, hearing, etc.) and unique manifestations of existing things (such as gold). However, I have never actually seen a Pokemon game delve into the concept of a colour-based type. If I could make a new type, it'd be the 'Chroma' type, and would revolve around the manipulation of colours. For example, an attack could involve the mixture of two colours representing two types of elements. A red-based attack could inflict burns on the opponent, while blue could cause freezing. This may sound quite overpowered with each colour connecting to another type of attack, however I'd weigh it down with some significant weaknesses and not too many seriously overpowered Pokemon (I also would try not to make it the region-centric type like fairy was in XYZ, because having a region focus on one type primarily is boring).
Chroma > Rock
Chroma > Ground
Chroma > Water
Chroma < Fire
Chroma < Fairy
Chroma > Electric
Chroma > Ghost
Chroma < Psychic
Chroma < Dark
Chroma < Grass
Chroma - Steel
Chroma - Fighting
Chroma - Normal
Chroma - Flying
Chroma - Ice
Chroma - Dragon
Chroma - Bug
Chroma - Poison
I also kind-of feel like if Chroma was to be added to any already-existing Pokemon's types, it'd probably be a secondary type, not to replace the Pokemon's first type. I don't remember off the top of my head any Pokemon who'd be a glaring fit for this type, so Pokemon with only the Chroma typing would have to be mostly brand new. Same with the moves, only a select few current moves would work well with the type, besides them it'd probably be all new stuff.
Unlike types like Fairy and Dark which have a general association to respective allegiances (fairy tends to represent good while dark represents bad), Chroma would represent quite a neutral perspective, much like the three starter types.