I remember you showed me a picture of the crystal set re-shaded in gold. That I would call extra set worthy, as it changes the whole atmosphere.
I agree with that, particularly given that new-format NL supports, and designers frequently use, tileset mixing. The "gold crystal" tileset would have a different set of tilesets it goes well with. I don't think it should be added right now just in case anyone ever wants to use it, but I think if anyone
does come up with a level where the "gold crystal" set is needed to give it the aesthetic they want, they can add the tileset if they wish.
Another point is that NL already has a large number of tilesets, so adding one more doesn't make the task of finding a particular one in the scrollbar very much harder.
This is why I support the current situtation with the Genesis sets: Genesis Dirt (completely different coloration) is its own tileset, the others are not.