I lean towards names that are youthful and informal but aren't twee or overused. Obviously everyone's idea of twee, youthful and informal is different so I'll give my list so far and hopefully that can help give the gist
Maria
Lucy
Summer
Alice...
Although I understand the appeal of Elizabeths and Carolines I find them a bit stuffy (speaking as a Katherine). Although I adore Maria it is extremely popular in many Latin countries so it probably won't be used... maybe MN though