I don't know how many times THIS has been done with BYOND, but it'll sure present itself as an interesting twist to combat. Giant worms, like from Dune, that can burrown in and out of walls, and basically create walls with their bodies. I'm thinking about adding some kind of "surround" AI, for them to trap their prey. This is of course going to extend now into large mobs taking up more than one tile, such as gelatinous cubes and horses. With a little planning, you might tackle a monster easier by taking advantage of the surrounding environment and your different sizes.