I just had some idea about this and looked for something similar here.
The only related idea is this, maybe it is even better combined (but the link is for 3.5) : http://paizo.com/threads/rzs2iuei?Elemental-of-Surprise#1
element of surprise / sneaky metal
This dull black metal charges itself with surprise over time, which can be used to the advance of attackers wielding items made of it.
When attacking an opponent with a weapon or ammunition made from the element of surprise, the target has to make a will save (SG 10 + number of sneak attack dice of the attacker + the attackers WIS-modifier) or countin as flat-footed for this attack. This is a mind-affecting effect (confusion), against whicht characters with the uncanny dodge ability are immune. The magical enhancement bonus of the weapon or ammunition (whichever is higher) is added to the DC.
A weapon can be used 5/day it needs to charge again, ammunition can be used only once.
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Something similar could also be implemented as some kind of rogue trick...
I didn't think about gold values or other applications yet.
Suggestions?
