I can definitely understand the appeal of an evolving nightmare, different every time you see it, but you hit the nail on the head with the bit about time spent on art assets! Reusing things might evoke feelings of familiarity, so it could work. Though everything's still in the relatively early planning stages currently, so we'll have to see how everything works out!

Some of the nightmare battles in my current plan are unwinnable, though, with defeating the nightmare being an important end goal. Though too many unwinnable battles wouldn't really be fun, of course.