I don't know if any of you out there remember the Incantatrix PrC from 3.0, but I would love to see something like that become available in Patfinder in one way or another. The basic concept is of a metamagic specialist who can enhance the abilitis of allied spellcasters. The most compelling of their abilities was how they could ready an action to apply one of their own metamagic feats to an ally's spell as it was being cast. It wasn't easy, requiring a DC 15+ (3*adjusted spell level) Spellcraft check in order to do so, but I loved the idea of it.
It had a smattering of other cool abilities, somewhat similar to a spellthief, but feats and or class abilities that allowed for a true metamagic specialist and the ability to modify an ally's spells as they are being cast was an ingenious idea, in my opinion, and I think it would rock if Paizo added something like that. Maybe as a feat that requires a minimum of 5 metamagic feats as a prereq. or something like that.