Have to agree with what's been said about Lucy. She's technically got a lot of power, not only with her spirits (and possible intelligent use of them), but she's skilled with that whip, too. So she pretty much has to be kept dumb in order to manage that power level.
One particular thing I thought recently, is that there is technically a 13th zodiac sign:
Ophiuchus, also known as Serpentarius. It might be an interesting trick to pull, having Luzy discover the 13th key after she gets the other 12.
But yeah, she's been shown to be getting stronger, too, with being able to summon more and more spirits at a time. If she ever gets to a point where she can bring out all 12, she'll be a serious threat.