Honestly, depending on how tricksy to implement, but having (for instance) a 32x32 greyscale bitmap be a custom brush tool would work here. 0 for max push-down, 255 be max push-up. 127 be a no-change.

Let region and parcel owners get real imaginative with their build brushes and go ham with it.

A circle would be nice! Imagine terraforming a crescent moon lake with the tools we currently have... This would have been helpful without having to modify a .raw file or having this as an option if the region is not yours to edit that feature.

It would also be awesome if we could set this via script (llModifyLand) using mesh/building prim physics to make any shape we want as the terraform area!

I really hoped for something like this when building my pathways system, which includes arced paths, so I could just allow terraforming from the path itself, to follow how the pieces are configured. It would be nice to be able to do more than one square area at a time... I had to come up with a work around for my terraformer tool.

Can not see any advantage, the land tools are so basic that they are akin to painting a portrait with an industrial sprayer.

the best option is to be able to set a prim(s) to work as a land deformation tool

or better, a spherical option to make a round bottomed depression

I personally would like to see a tunneling ability to ground terraforming

YES, a circular area would be nice. What would be better though, is a finer setting, so that you could terraform a smaller area than the leftmost size slider setting currently permits.

I would even go so far as to give you a choice of shape for the target area. On mainland or rental plots, a square shape is very useful, especially at the edges, even if you need/want fixed straight lines in land design, but for the rest, a round selection is much better.

This would be AWESOME, I have wanted a circular option when terraforming for SO long.