I had already planned for them to fight an Aboleth, actually.

The BBEG is interesting. He's a Daelkyr spirit that's been stuck on the Material Plane since the Daelkyr War 10,000 years ago who survives by possessing body after body. So his abilities change based on what body he's in; he created the demiplane when he was in a powerful wizard, so there could very well be illusions. The question is how to use them creatively and not just have will save after will save.