I mean-
Being honest, we don’t know if they stopped with the rock type.. after all, the gen 8 fossils are incomplete, all of them are missing 1 type each, so I’m pretty sure they’re all going to have the rock type aswell. But yeah, I agree, spino should get a fossil... unfortunately, I can only see it as rock/water or water/rock and/or rock/dragon or dragon/rock... which are typings that are already used... though a therizinosaurus sounds cool, like grass/rock?