The game engine can only support eight textures at one time as far as we know. In the Planet Coaster days they eventually added a scenario editor which allowed you to pre-select eight textures for your map, but that was the limit. There was a recent update to the engine, but whether this limitation has been overcome or not remains to be seen; personally I'm doubtful, but I do think we will one day see a similar scenario editor added to Planet Zoo.
As to whether they'll add more textures, well, "savannah grass" exists already if you play on a grassland map, at least the stereotypical dry-season yellow grass does (I personally don't really like this, as the African savannah is often green), and red sand is also accessible if you choose to build your zoo in the Oceanian desert.