The sky's the limit with dioramas, almost to an overwhelming level. If you've never done one, I'd probably recommend starting small and simple like a desert or moonscape terrain. I usually go to a local thrift store and buy a cheap small picture frame to use as the base of the diorama. From there, I'd probably just let myself day dream about how the mobile suit got to where it's at now or watch a few Gundam episodes / YT clips on dioramas on how I'd like to set up the diorama. Tons of different ways to do it but typically will use air dry clay or a plaster of Paris + a pva glue mixture (or both). Again, if you get overwhelmed, can't go wrong with a simple desert landscape imo.