It would be nice if we could give a start and end ground elevation.

Sloping terrain would be a nice feature that would have helped my mountain project initially. But, as you have probably figured out, there are several workarounds. See this link (4817) and more.

It would also be nice if we could give sloping walls not only from the top but also from the bottom.

For those rare occasions, don't you think the (fairly) straightforward workarounds will suffice? Such as:
- Drawing a sloped wall, export as OBJ, import and rotate.
- Using an invisible triangle (or whatever) imported as a window/door to make a cutout in the wall.
Just my 2p worth. ok