I think you need something short to balance it, and not beginning with a vowel (as then the 'a' on the end of Isabella tends to run into the beginning of the next name and you get something sounding like Isabella Ran for Isabella Anne, for example. Might also be a bit much if the middle name also ends with an 'a'.
Isabella is a lovely name, and very classic, so to fit in with that how about Isabella Mary, Isabella Catherine/Katherine or Isabella Jane?
Isabella Rose is lovely, but with Isabella you have got the added complication of it sounding like a question 'Is a bell a rose?' IYSWIM.