I like [name_f]Arista[/name_f] and [name_f]Zella[/name_f].
[name_f]Arista[/name_f] has a quiet kind of magic to it – distinctive and arty but not over the top! Also nickname [name_m]Ari[/name_m], [name_f]Isa[/name_f], [name_m]Issy[/name_m], [name_m]Aris[/name_m], Ris, etc. have that same earthy feel as [name_f]Fenna[/name_f]
[name_f]Zella[/name_f] is zippy and energetic and while I do think it could be misheard as [name_f]Zelda[/name_f], I think [name_f]Ella[/name_f] and [name_f]Stella[/name_f] are familiar enough that the sound is recognizable?
Of the two, I slightly prefer [name_f]Zella[/name_f], but both work! I do also really like [name_f]Petra[/name_f] 
If I were to suggest some with the lovely Fenna…
[name_f]Katri[/name_f]
[name_f]Asta[/name_f]
[name_f]Suki[/name_f]
[name_f]Vita[/name_f]
[name_f]Sol[/name_f]
[name_f]Bina[/name_f]
[name_f]Saskia[/name_f]
Pepsa
[name_f]Runa[/name_f]
[name_f]Delta[/name_f]
[name_f]Leonie[/name_f]
[name_f]Suvi[/name_f]
[name_f]Sigrid[/name_f]
[name_f]Twyla[/name_f]