Here's some underused tilesets which could really use some levels made with them if you feel like being original:

Grove Shrine - LRK
Coral Coves - Violet CLM
Chateau - Toxic Bunny
Some JJ1 tilesets - Various Authors
Beach/Beach + - Epic/Purplejazz
Colon - Epic
Inferno/Inferno 2.0 - Epic/P4ul
Persian Paradise - Disguise/Mirrow
Unreal Imagination - Disguise

There are loads more underused tilesets but these are a few suggestions.
Some of them are very hard to use but its worth a try.