I just realized what big things Demon is missing: terrain and level themes (not sure if theme is the best word for it)! Take for example all the terrain and obstacles in DCSS: doors, shallow/deep water, plants/fungi, lava, trees, clouds, transparent walls, and other stuff I might be forgetting. These all add another level of tactics to combat (in the form of positioning) and can make each level seem more unique. As of right now, there aren't really any terrain features other than floors, walls, and portals. Also, each level could be shaped in certain ways to make each level more unique compared to the rest (having open levels, narrow corridor levels, room based levels, water heavy levels, and so on). I think lacking these features is a major reason why each level of the tower (and every area WITHIN each level) feels and looks practically the same.
In addition to making levels more "fresh", terrain can interact with certain demons and skills in interesting ways. Some situations that might arise are flying demons being able to fly right over water without being impeded, using shallow water to hinder land demons, lava melting ice attacks, pounce attacks being able to leap over obstacles, smite-targeted skills gaining extra utility when paired with obstacles that don't block line of sight, and so on.
Adding terrain could also serve as the basis for new skills that interact with or modify terrain, like digging, plant growth, freezing water, creating "shadows" (dark clouds) that can impede LOS and light attacks, or using a light skill to extend LOS. Some of these potential interactions might take away from Demon's encounter-based combat system (although they could still be very useful in combat). I'm not sure what your design goal with Demon is, so maybe allowing out of combat actions isn't on your agenda.
Anyway, I think adding different types of terrain and different types of floors would make levels more interesting and help improve the replayability of the game.