Seeing how we already have a feat in the original Complete Scoundrel that let's us reroll any failed save a feat for either a fortitude, reflex or will save seems kinda weak...
Sure, the original could only be taken at level 9 or later, but perhaps you could add a +2 or +3 bonus to the reroll.

Lucky guess would be more usefull - and it would fit in flavour - if you ccould guess wildly on untrained knowledge checks, perhaps in exchange for one luck reroll spent.

Serendipity is just weak. +1 to AC if you do not use all of your precious rerolls?
I'd increase the AC bonus as you either gain more levels or better yet more luck rerolls, and/or add a save bonus to it =)