I tried searching and found some mentions, but not quite a topic like this... So, here goes.
I'd like an option to build and flatten hills. You might need some new skill to perform these actions.
To remove a hill, you'd perhaps need some advanced digging skill/tool. Maybe you'd just need a shovel and be unable to do it by hand. If it's a shovel, maybe you can right-click hills while it's equipped to get a "Remove" option. If it's a skill, then I guess you'd click on the hill with the skill. Now you'd be filling up your inventory with soil. The hill would have some set number of soil in it and you'd have to keep at it until it runs out. Perhaps you could use prospecting to see how much soil is left, I dunno. To prevent hills from being removed too easily (in case they were being used as natural defense or whatever), perhaps they'd have a minimum of 1000 digs before flattening... maybe a capacity of 5000 or something and the amount is randomly generated when the hill first came into existence (or if/when this feature was added).
To build a hill, you'd start up a little construction site and have to add at least 1000 soil to it. Perhaps you'd have to also pound the soil down, maybe add water, leave it be for a while, add more soil, pound it down again, etc. until the hill is a solid hill and not just a loose pile of dirt. This could also involve clay, I dunno. Perhaps it could also be vulnerable in this state... like, people can attack it to scatter/destroy the soil, and even destroy the fresh hill if they keep at it long enough. Destruction could be significantly faster with a shovel.
And all this effort is just to add/remove a single tile of hill, mind you. I dunno if additional hill tiles should be easier to build like with fences. If so, I suppose they'd be able to hold less soil than the origin the farther out you go so that the hill is a hill, not a platform... but they should still have a decent minimum amount in them so they aren't simply treated as weakpoints... Though I suppose they realistically would be, but I suppose people would also build on top of hills rather than hoping their enemies don't simply go over them.
And maybe you wouldn't be able to build hills like this on certain terrains (mountains, caves, swamps, whatever).