Funny this should be mentioned, because I visited a school today where I might potentially work next year, and there was a Year 8 student carrying around a load of helium balloons her friends had given her, saying "happy 12th birthday
Marnie". It's not a name I have ever seen used in real life before, but I do quite like it.
Mn suggestions:
Marnie RoxanneMarnie LianaMarnie SusannahMarnie FrancescaMarnie LouisaMarnie TeresaMarnie Corrina
S xx