I think it would be neat to have a metamagic feat called something like "Unseen Spell"
-Prerequisites: Still spell, Silent Spell, able to cast 2nd lvl spells.
-Description: The caster has mastered the ability to cast their spells with no more than a thought and unbeknownst to any onlookers.
-Benefit: No somatic or verbal components are required and there are no visual effects of the spell being cast.
-Level Adjustment: An Unseen Spell takes up a slot 2 levels higher than normal.
I'm not sure how possible this would be mechanically, and whther or not it the visual effect could actually be seperated from the spell, so if that is the case....
I suggest a metamagic feat that simply combines Silent and Still Spell for only +1 spell level due to the cost of already having used two feats in order to gain the prerequisite for this 3rd feat.