I like that idea - can certainly cook up some appropriate creatures.
I'll probably keep some of them though. The idea of a cleric's look of horror as they realise they've bound an intelligent servant of their deity into Ravenloft seems too good to pass up.
I'm starting up a Ravenloft campaign soon, beginning with Curse of Strahd, and something about the alterations to magic struck me. There's a very clear mention of it being impossible to escape from the Demiplane of Dread, and yet summoning spells are left alone to dismiss creatures back to their...