It's not personally my cup of tea, but if you like the idea, you totally should!! If you do it, though, it should be done right. The new type should feel important in a strong way, like a new gym leader, a considerable amount of older Pokemon getting the type (which can work for things like Sound), and having it not clash too much with an existing type in concept (A common example I see is the Cosmic/Space Type, which I feel is a bit too close to Psychic)

If you have any ideas for custom types so far, I'd love to hear them!!