You could code an entirely different game (say, Tetris) in Lua within PZ if you wanted to. Traditional total conversions (replacing all assets etc) isn't very practical since you can't modify the character models other than replacing skins though. Much of the game's mechanics are written in Lua (including the user interface) so you can modify or replace those, or create entirely new gameplay mechanics. Look through the script names in media/lua/client, media/lua/server and media/lua/shared to get an idea of what the capabilities are.
