I tried to describe my vision of hotkeying terrain tool mods. And that's what I've got:
The main reason for new hotkeys is sculpting. Hotkey for ingibitor mod is nice to have, but when I build, I woud like to focus on design, not on fighting with controls. That's whrere good hotkeys are essential.
In creative mode you change tool behavior in R menu, where you can change the brush size and deformation speed. But it's not perfect. I do not want to press R, then moving mouse, thinking about desirable setting value. It's easy to imagine how to change brush size without even knowing about it's numerical representation or which mod you need for it. Just scroll the mouse wheel with some keyboard modifier (or analog stick). Deformation speed has obvious feedback and if you missed desired level then adjust it again. That is more easier, than entering menu... trying value... nope... entering again... nope... again... Or even swapping mods, when inventory is full of soil canisters.
Hotkeying synergies with "true flat". I need an ability to adjust not only brush plane (Flatten Mode), but also brush angle, using some reference in the world. For me it would be perfect if I can copy/paste angle and relative position to align different parts of my project. Voxel grid can be this kind of standard. It would be great to have ability to make the grid visible with hotkey; automatically align the brush with the grid under different angles (45°, 54.7°, etc).
Astroneer can and definetely will beat minecraft in terms of creativity right after an intuitive and responsive UI brings the creativity in survival mode.