I think a cool miniboss could be a fly (which would respawn every 7- ish days.) Imagine if an old soda cup was lying on its side on the stone wall across the pond. The cup would have a straw out of it, which you could climb in up to get into the cup. Inside the cup, there would be some old soda and several ice cube platforms. One fly would be in there, which you would have to fight. The fly would be quite big, and would constantly be buzzing around the arena, making it a hard and challenging fight. It would charge at you, so you would have to jump from ice cube to ice cube to avoid the fly's attacks. If you did kill it, you could get a few useful items. One item would be a fly eyeball, which you could use for two items. One would be binoculars, which would be made from a fly eye, a clover leaf, some sap, and woven fiber. Another one would be a telescope. It would be made out of some weed stems, some sap, some spider web, one or two clover leaves, and a fly eye. It could be set down like a stuffed bug and would be able to be looked out of. I think this would look really cool on observation towers!