I know two little girls who have Annie as a nn. One is named Anouk and the other is Anneke (pronounced An-A-KA). Both these girls are adorable and both suit the Annie nn.
Anne is my all time favorite name! However, if you are only going to call her Annie, then I would name her Annie. Gorgeous on its own.
If I were you I would ñame her just Annie if I love it
If you looking for any longer version:
Aniston too trendy?
How about these? With NN Ana/Ani? (For a modern twist)
I'd suggest Anwen and beautiful welsh name that can get you Annie easily.
Most of my non-conventional favorites have been mentioned (Xanthe, Johanna [och, LOVE Johanna! I think it'd be fab with Alexander, Madelyn, and Genevieve!], Arianne, Hannah, Evangeline, Morgana, Anthea...), plus a load of other ones I really like (Anais, Anouk, Annika, etc.!). The only remaining two ideas I have are Giovanna and Angelina. I'm sort of afraid Angelina's a bit close to Alexander, though, and Giovanna is a bit close to Genevieve. Still, they're pretty and distinct enough to use imo.
I love Anastasia, Annabeth, Annaliese, and Miranda.
I've used Anaselia in one of my stories. I've also used Nelian, Arandia, and Elliana.