Hi there.
Here are my thoughts on your list:
Serena-This is a fine name, but it feels like a different style as [name_f]Savannah[/name_f], & at the same time has too similar of a sound to [name_f]Savannah[/name_f].
Charlotte-I don’t love the flow of [name_f]Charlotte[/name_f] [name_f]Michelle[/name_f], but not a deal breaker if [name_f]Michelle[/name_f] is an honor name. [name_f]Charlotte[/name_f] is nice in general, & I think works w/Savannah. A little more Victorian, but it works. Anyway, it’s way way popular if that bothers you.
Caroline-No, I like [name_f]Caroline[/name_f] [name_f]Michelle[/name_f]! [name_f]Caroline[/name_f] is sweet & goes well w/Savannah. Well known & simple, but not too terribly popular.
Harper-Meh, [name_u]Harper[/name_u] doesn’t do much for me. It just sounds a little boyish to me, honestly. And it’s super over done.
Karina-Not my personal favorite, but it’s quite nice & goes well w/Savannah. Simple & fairly unique, so that’s good.
Clara/Claire-Hm. I like [name_f]Clara[/name_f] more & it goes better w/Savannah. They both have a cute southern bell vibe.
Madison-Meh, [name_u]Madison[/name_u] seems very last name-y & trendy.
Dakota-I really like this for a boy. Not my favorite for a girl. I’d like it as a brother to [name_f]Savannah[/name_f].
Amelia-Ooh, I love [name_f]Amelia[/name_f]. It works well w/Savannah. It’s a great balance of bold, smart, feminine, vintage & kind. It’s very popular too, unfortunately.
Aria-This is okay. [name_m]Just[/name_m] kind of trendy & over done, so doesn’t personally excite me.
Here are a few more ideas that you may like:
Audra
Georgia
Louisa
Magnolia
Marigold
Olivia-very popular, but a great name.
Willa
Hope this helps. 
.