[@Banana] Mmm.. I dunno. Generally I'd discuss it with the player/go with the obvious.

For example. Diana would become a dragon-girl. SHOCKING. 

Marquis, I think, would become an owl. Which is the primary reason I want Artoria to get her hands on him.

Victoire, Artoria isn't interested in, because vampires always seem to end up being bats or wolves, and she's already got wolves and a flying form.

Grafdakka would definitely become a cat. No question.