I have been looking at options too. I think I am leaning towards the Trijicon HD's which are for the M&P series, but not specifically for the Shield, but still fit w a minor overhang. The XS Big Dots were designed specifically for the Shield if you feel like stepping away from the traditional sight picture. TruGlo TFO's are another option that has both a tritium vial and fiber optic in the front sight but they are long sights and may take away from the already small sight radius of the 3.1" barrel.
Apparently APEX has made a Shield carry/duty trigger system for the Shield, however I'm not a fan of modifying the trigger on a carry or defensive gun. That is a decision only you can make. It's said to drop the trigger pull to around 5 lbs or so.
That's all the info I got for the neat little pistol!