I just figured I could toss out a few total overhaul ideas that might be possible, but that I personally aren't really that interested in. Could be a fun experiment for modders looking to work on big projects I guess, and might inspire people. Currently I only have three such ideas but I might add more later if there is interest.
(I should clarify I'm not an ideas guy asking for people to make a mod for me, I mod a lot. These are just ideas that I'm not too interested in and think someone else might like so I'm tossing them out here for those who might feel inspired.)
1. Underwater overhaul
Basically, you turn the dwarf fortress world into an aquatic one. You can accomplish this by removing all the land animals and birds, and giving fish the ability to breath air, live on land biomes, and fly, giving the impression of being "underwater". Then you just add in the civilized races of your choice and replace the grass/trees with seaweed, coral, ect. The actual water becomes super-dense brine pools that can have eldritch horrors lurking underneath. Maybe an emphasis on biotech (custom materials and crafting) because smithing doesn't work so well underwater. Different biomes on the land can be converted into different underwater ecosystems.
2. Bug overhaul
Delete everything but bugs, and make the bugs huge. Well, people sized. Then replace all the grass with dirt (grass doesn't need to actually be grass, it can be anything you name it) and change the trees to mushrooms, fungi, and blades of grass. Megabeasts are shrews and lizards and such. Maybe delete metals and replace them with wood and stone embedded in the ground that you can mine and forge into weapons. If you're going the supernatural route pixies and fairies could be playable races alongside the bugs.
3. Dune Mod
Some guy figured out how to make sandworms with DFhack sort of (messing with gas trails to great the illusion of a multitile creature basically). So you could do that. Dune even fits DF since melee weapons are the norm. Problem is 1. it uses DFhack and i don't like relying on 3rd party utilities and 2. You would have to create a custom world that's all desert and I generally don't like restricting players to a single biome. I may actually try my hand at this one but probably not for a long time so feel free to give it a shot yourself. Maybe a sort of battle royale alternate universe where all the factions settle down on Arrakis to duke it out for control of the spice. Plenty of variety for factions, not so much for fauna.