Not really. I think Imp Crit and Keen should stack with each other and any other effect should emulate one of them (and therefore not stack with the original).
Keen Edge, Scabbard of Keen Edges and any prc effect that is called "keen" is a keen bonus.
Anything that refers to critical hits, improving the crit range, improving the threat range is considered a critical bonus.
Thus there are a total of 2 effects that can affect a threat range which stack together but not with anything else. There could be other special effects from prcs that could provide "enhancement" bonuses to the threat range, like the Weapon Master prc, but these are of course optional.
Technik