Some of my favorite level designs in FPS games are the ones that stick out in my memory for being so different. A great example of this are the "Abstract" maps created by map maker Nipper for Counter-Strike: Source. The maps commonly feature brightly colored textures, various song snippets, and of course abstract world geometry to create an otherworldly atmosphere. There were full packs of these kinds of levels made for mapping competitions in the late 2000s. These kinds of maps really make for a memorable playing experience, and there are enough of them out there that you could have a server with the whole map rotation being these.
My favorite of these maps is the one titled "Zuhh" seen here:
https://youtu.be/2l1nZZ12_Wk?t=437