Here's a sample adaptation of the coincidental magic idea. It was part of a larger project I worked on called "arcane taboos."
Reduced casting refers to those conditions under which you function as a spellcaster 2 levels lower (or half your level, whichever is the greater penalty). Your spells per day, your caster level, the maximum spell level you can cast – all these are reduced as if you were a wizard 2 levels lower than you actually are.
Prohibited Casting refers to those conditions under which you must make a (DC 20+spell level+caster level) Will save to cast spells. You are fatigued after attempting such spellcasting, whether or not the spell is successfully cast.
Coincidence Mage
Your magic depends upon you remaining unobserved. The laws of reality are shaped by a collective consciousness. The presence of others seeing your magic calls upon the collective disbelief or condemnation of such things by the greater community. Only your great strength of will overcomes the mundane shroud that covers humanity’s eyes to the truth of magic that surrounds them.
Coincidence or Magic?: You gain the coincidental magic feat (see sidebar below).
Reduced casting: When observed by a small group (1-10) of people who disbelieve in or condemn magic, when a wizard sees through your coincidence magic, and whenever you cast a spell with an obvious effect that would be witnessed by someone who doubts or condemns magic.
Prohibited casting: When in a large crowd and when casting against priests.
Special: Casting "wiz-bang" types spells in plain view is hazardous for your reputation, and possibly your life. For every 6 spell levels worth of "loud" magic you cast in plain view in a populous area, you suffer a -2 penalty to all Cha-based checks in that community. Likewise, base reaction toward you in that community decreases by one step (e.g. indifferent to unfriendly). Should reaction toward you shift to hostile, a lynch mob forms to burn you for witchcraft, heresy, consorting with dark powers, atheism, etc.
COINCIDENTAL MAGIC [Metamagic] [Trained Only]
In order to remain hidden, you have learned to disguise your magic with coincidence.
Benefit: When you cast a coincidental spell, those who observe the effects of the spell assume it is
simply extraordinary coincidence, but not magic. When describing the coincidence you seek to emulate, the GM will determine whether it is likely (DC 30), unlikely (DC 20), or far-fetched (DC 10). Obviously not every spell can be couched as coincidence, and the GM will have to arbitrate. A wizard observing the coincidence can attempt a Spellcraft check to see it for what it really is – magic! When casting the spell, you may decide to delay the coincidence for up to 5 rounds after the time you cast. A coincidental spell uses up a spell slot one level higher than the spell’s actual level.
Special: Unless you take other feats to disguise your spellcasting (e.g. Still Spell, Silent Spell), you must perform this feat hidden or otherwise out of sight. If others realize you are casting a spell, they connect the coincidence that results to your waving your hands, gesticulating wildly, and pulling out strange materials.
