I am in agrement with the keep it simple crew. A light is nice, but your gun can no longer be used for such simple things has hunting. Sidesaddles have alot going for them, but you can get most of the same benifit from a butt cuff. Mag extension..if you think the threat is high enough I suppose so. That said a know at least two high end instructors who's HD guns don't have them.
The two additions I am highly in favor of is a trinium front sight. It is a good aid in indexing your gun in the dark and last but not least a sling.
If you are building a gun for serious offensive opperations then go for this:
Desert01