GZDoom adds loads of engine features like 3D floors, transparent water, etc. ROTT:LE will be the definitive version of ROTT for all time most likely, it should receive similar upgrades to boost longevity with the mapping scene. I know it's just KEX underneath it and technically it could be made to do anything, but I feel there are some modest improvements that would stay in line with the original engine's aesthetic.

Someone else mentioned a cut rain effect. I'd like to suggest the ability to vary floor/ceiling textures on a per-tile basis, with different footstep sounds depending on texture. This would permit levels to have a lot more visual variety. Indoor levels could have skylights. Outdoor levels could have overhead structures. City levels could have streets and sidewalks, with sprite based street lamps.

This goes hand in hand with the suggestion for custom asset pack support, but I already suggested that in another thread.