I asked about that in the last AMA:
This was their response:
To which I responded with:
Sadly, nothing more was said on that topic.
Regarding multiple material variants:
Are there any plans to allow developers to use different material variants for terrain simultaneously? Currently, you are limited to using only one variant of each material for the entire game. Additionally, will you also enable us to paint the 3D grass over any material, rather than just the grass material?
Thank you for your question. We’re currently focused on improving our terrain’s overall performance and scalability. Internally, we’ve started to think about the future of materials on Roblox. This is a great request, and we’ll take note while we explore the best solution for all creators.
For a full summary of all questions asked and answered, you can read more here: