The name Isabeau has been growing on me for a while. It was the name of a French queen, Isabeau de Bavière. I think it sounds classic and beautiful, and it would be a good alternative to
Isabelle. I have a sneaking suspicion that it was once a masculine name, though, as nouns that end with -eau are masculine in French, and "beau" is the masculine word for "beautiful", while "belle" is feminine. Anyway, that hasn't put me off it. What do you think of Isabeau?