Abandoned villages

1. Add more ruins looking like decayed buildings from vanilla villages.
2. Place these just as if it was a village, with a dried-out well generating naturally and the ruins as sub structures around it.
3. Stone variants for temperate and snowy biomes.
4. Stone and Sandstone variants for deserts, half buried in sand.
5. Overgrown variants for jungles - the nature got it back!

Sunken structures

1. Villages and other structures lost in the ocean.
2. Sunken ships.

Trees

1. Treehouses.
2. Gigantic trees with a hollow trunk.
3. Petrified trees.

I'm getting warm with tinkering with this mod, maybe I'll create some of the structures when I find the time...

1. Doesn't seem like a bad idea - in fact I would probably have already done it if it were a little simpler. In theory it would be as easy as injecting a ruins transformer over each generating structure and preventing villager spawns - but unfortunately that's currently not realistic.

2 and 3) Yes please! I'm very happy to add stuff like this, and while I do not have the time to build them myself usually I do sometimes crawl the existing structures in search for usable things. So might happen, but if you're ready to create I'd be very happy as well :)