I like
Petra. It's clearly feminine but isn't too frilly or insubstantial. It's simple to spell and pronounce and not too popular, but not too out-there either. It fits my criteria perfectly!
Petra JuliannePetra LouisePetra MiriamPetra JosephinePetra MariannePetra CarolinePetra Susan /
SuzannePetra Jane