In minecraft earth we should get some more challenging boss mobs in legendary crystals, that are yet to come. They should be bigger than normal mobs and have an healtbar because a boss needs to be big and recognisable as a boss, thing that the current mob used more as boss (the skeleton spider) lacks. Also the bosses shouldn t be too large because they would need to fit in the AR experience, so The Ender Dragon and The Wither that occupy a lot of space and cause damage in large areas wouldn t be ideal. So my suggestion is that the boss could be a ravager that could be unique to Minecraft Earth using a pink colour, higly recognisable. And after defeating it, it could leave an egg that could be hatched to get the mob itself in a buildplate maybe in a baby form more suitable for buildplaes.