I would like to bump this topic as it seems very important to terrain creators and game developers that have terrain inside their game.
Having a fill option inside studio for terrain is a great idea and feature for terrain creators and much easier to have a certain material fill an area inside a certain area that has many holes or pieces that need to be filled with either a liquid or solid material. A fill bucket tool for terrain and maybe parts would actually be a great way to optimize time for terrain like you have up there as an example with the select tool as it cuts through the original terrain of the given area that you need to fill.
Anyway, having a bucket tool is more efficient than spending time eroding and growing certain areas of the given area that you need to fill which might take more than 10-12 minutes, which with the fill bucket can take up to seconds!
In the end, I support this feature 100%! 