Wren is a family surname for me, so I'm very biased. But I do think it would make a terrific name for a girl; off-beat, yet entirely respectable.
My favorite combination is
Wren Victoria. I saw a Victorian drawing of a wren once, very Christmas-y with snow and red berries in it. It stuck in my mind, so
Wren Victoria feels very natural for me as a combination. Other ideas:
Wren OliviaWren Emilia /
AmeliaWren AureliaWren IsidoraWren Seraphine Wren AudreyWren Eileen
Keep in mind that
Wren could also be a nn for
Renata,
Katherine /
Catherine /
Karen, or anything you can think of. :-)