Maybe this would be a good time to discuss Spider's Lair in general, not just the possibility of changing up the monsters, but also the layout of the level. (For difficulty purposes I'll assume UV enemies.)
Right now we have eighteen Arachnotrons and a bunch of teleporters that lead you around the map. Entering a teleporter takes you directly TO a spider, although it's often dispatched quickly. The rest of the level is sitting on that side of the map, preferably behind cover from the rest of the map, shooting spiders as they teleport in. Even on UV, with a rapid-fire build this is relatively trivial. On just about any other build, the lack of cover and numerous enemies make it very difficult to make progress, especially when you consider there is no health until the very end.
Now, don't get me wrong, but Revenants are ridiculously difficult without cover, and Mancubus can be EASY without cover. Also, Fireangel builds would make these places not only trivial but highly desirable (what with the gratuitous amounts of experience from Rs and Ms). I'm not sure we want to just change up the monsters and call it a day. Might be better to include some actual cover, for instance, and switch up the teleporters to appear it less predictable areas. If we can make the level more interesting, we can work with new enemies, but at the moment Spider's Lair needs an overhaul before we unbalance it to this extent.