I like
Isabel and especially
Isobel a lot. Neither of them seems to need a nn, but
Isabella is inconveniently long and complicated, so it would very probably shorten to
Bella, which I find very dated on a human and very popular on domestic pets.
I was at Uni with two girls who were inseparable friends, both dark and pretty and Drama students, and were both named
Isabella. They were known for convenience as
Bella and Iz. I really don't enjoy any of the nns.