Molly is originally a pet for of
Mary, so I guess all
Mary names would do -
Mary,
Maria,
Marie,
Marissa,
Miriam,
Moira, etc. - and names starting with "mo-", such as
Mona,
Morag,
Morna,
Morgan,
Morgaine,
Moreen, and
Monica.
You could also use
Molly as a MN and refer to her as
Molly anyways, if you're afraid that she's going to grow up thinking that
Molly is too cutesy for a first name.
I personally don't think
Molly is too cute to be taken seriously, though. It's adorable, but not babyish or anything, IMO :-)