It bugs me when people do that😂 my name is Rebecca and I know some spell it Rebeckah... Whyyy?!
Do you mean Rebekah? That is the original spelling. Rebecca is the anglicised version. My sister is Rebekah and it's straight out of the Old Testament.
On the other hand, if you do mean Rebeckah I'm totally with you, that's a travesty! 
I feel like you need something really traditional and elegant to go with Elizabeth and Sophia. If you go with a modern name she could feel cheated or different. Even though Sophia is so popular right now, it's still a classic, imo.
Elizabeth, Sophia and ...
Annabelle, Beatrice, Charlotte, Delilah, Florence, Georgina, Harriet, Imogen, Joanna, Katrina, Lydia, Miranda, Natalie, Olivia, Phoebe, Rosanna, Thalia, Victoria