Izzy is sweet! I’ve always liked her for Elizabeth and Felicity in addition to Isabelle. I think she’d also work for these:
- Louisa
- Isisa
- Isolde
- Sicily/Cecily
- Zia
- Aziza
- Isabelline
- Ysaline/Isaline
- Isaura
- Isannah
- Anneliese
- Giselle or Gisella
- Lisette
- Tamsin
- Wisteria
- Yasmin or Jasmine
Some of these are a pretty big stretch, but I think they all could work! I personally wouldn’t use just Izzy, but I agree there’s no reason you can’t. It’s up to personal taste, and I prefer the versatility of a full name with nn options. Good luck!
ETA: Also—love the suggestion of Izella/Ysella which was mentioned! I love her.