Ohhh, I love [name]Isabel[/name]! [name]Isabelle[/name] is my favorite girls’ name (I prefer the French spelling, personally), and the popularity doesn’t really bother me. My combo is [name]Isabelle[/name] [name]Ayelet[/name] [name]Grace[/name]–I think the uncommonness of [name]Ayelet[/name] balances out the popularity of [name]Isabelle[/name] and [name]Grace[/name]. 
I think many of the suggestions were really great–[name]Annabel/name was my first idea to suggest, followed by [name]Elise[/name]. They both have the fairly prominent -el- sound (or -belle sound) that [name]Isabelle[/name]/[name]Isabel[/name] shares.
I don’t know if they’ll really be your style, but [name]Arianne[/name], [name]Olivia[/name], [name]Charlotte[/name], [name]Emmeline[/name], [name]Rachel[/name], [name]Liliana[/name], [name]Hannah[/name], [name]Alice[/name], [name]Annabelle[/name], [name]Eleni[/name], [name]Caroline[/name], [name]Annika[/name], [name]Leila[/name], [name]Zara[/name], [name]Kaia[/name], [name]Ivy[/name], [name]Gwendolyn[/name], [name]Genevieve[/name], [name]Sabina[/name], [name]Eliza[/name], [name]Lyra[/name], [name]Ella[/name], [name]Sophia[/name], [name]Violet[/name], [name]Aurora[/name], [name]Julia[/name], [name]Magda[/name], [name]Willow[/name], [name]Valentina[/name], and [name]Verity[/name] are my other favorites, atm. I’ve also been crushing on [name]Lydia[/name] and [name]Fleur[/name] today… Some of these names are even more popular than [name]Isabel/name, so I’m not sure they’re much help to you, but I feel like the style is pretty similar. 
I think [name]Mirabel/name, [name]Christabelle[/name], etc., are similar in style, and I love the suggestion of [name]Adabelle[/name]! I would probably do [name]Ada[/name] [name]Belle[/name], as two separate names, but I think the name/combo is fabulous. 
Good luck! I hope this helps!