I love the Maskling changes, also it can allow the opportunity for new enemies to defend their own biomes (like giant spiders protecting rocky regions, etc.) or something like that. I think this helps give the masklings some personality, they are just killers they are protecting their people!
Biome ideas: alongside the biomes we see already
Desert: gives a use for Sandy ground other than coastlines. Could be rocky too
Swamp: very fertile soil but tons of shallow water everywhere. Making it difficult.to settle
Mountains; very rocky, with only small path ways of grass to navigate through. Maybe rocks have height? (Like two or three levels that slow movement even further?)