I used to like
Bella a lot, on its own. And then Twighlight and the Charge of the
Isabella Brigade took it over. So it need a serious rest.
Belle, on the other hand, still sounds fresh to my ears. It can stand on its own (stock:
Belle Ariadne,
Belle Myfanwy), but like you I prefer it as a nn. It wouldn't have to be predictable like
Isabelle,
Christabel,
Arabella, Mirabel,
Belinda, etc. It could be for something more subtle like
Beatrice Eleanor --> B
El -->
Belle.