Ooh I’m guessing it will be something Italian which is commonly used in England.
Victoria or Isabella would be the safe choice (works in both languages and has royal associations, Victoria is also one of Eugenie’s middle names and Isabella is a variant of Elizabeth)
Maybe Seraphina to honour Granny/Elisabetta for Great Granny/Filippa for Great Grandad
Alessia/Alice - traditional royal name/Philip’s mother’s name
Alessandra/Alexandra - Queen’s middle name
Elena/Helena - one of Eugenie’s middle names
Emilia/Amelia- Georgian
Liliana - nod to the Queen although maybe a bit to similar to Lilibet
I’m hoping for something along the lines of Valentina/Vanessa/Giulietta/Theodora myself!
I’m hoping for a less conventional middle name in line with his son’s unusual middle name. Hopefully she can have a cute little nickname too!