I have an idea why not a stoney boss, its attack moves are putting his arm on the floor and pulls out a huge chunk of the floor and throws it at you, making a hole as well, another move is that it will rush you full speed crashing the walls and droping rocks wich can summon the gemstone flies. It has a spike body, you can't hurt him with your tears and it will try to suck you in to his spikes. The only way to defeat him is throwing bombs at him when it strt sucking you in, if you don't have bombs thats fine becuase when he rushes to the walls he will also drop rocks that are bombs which you can grab and throw at it when he sucks. This can be a great idea for a boss since the stoney are pretty much useless sure it may push you but thats it, it won't do nothing else and you barely get damaged from other monsters, the only time you get damge by a stoney is by a spike, fly or spider, or fire, this boss can represent that stoneys are a force to be reckond with ( though they aren't), it can be a special way to summon it or be via random encounter, you may even creat a new item like a stoney mask ( which makes stonies invisible to you ) or a gemstone fly buddy ( where it changes to random gemstone flys and depending on which type of gemstone it may have different abilities). it just and idea but I see it as great potencial, this mod is so good and has a lot of potencial, like a lot of people want this to be a floor or a secret floor but I like it like this very simple, this mod might even rival some mods like alphabirth or godmode if you added something more, but either than that I love this mod, its one of the best I played and I played alot with different ones, this mod is in my top 5 favorite mods ( Number 1 being Antibirth, you understand right) and I hate it to go to waste, so keep the good work you might be one of the best modders in issac ever.

Also the rock boss appears in delirius fight along with hsi second, form and to top it all it fits