Honestly I like the 'assassin' talent that they had in DOS 1 before they removed it. it made it so any weapon could back stab. I think daggers and knives should be able to back stab without even having to use a talent point, and Back stab should make it so any one handed weapon can back stab.
It was removed for a good reason. As Neonivek said, it makes the archtype of a dagger-using Rogue pretty worthless when you can take your big tough fighter and do a ton more damage without any of the downsides.
This is especially notable in D:OS 2 which has flattened AP costs and AP points so that basically all attacks use 2 AP and all characters will get 4 AP a turn. That has already reduced the niche Rogues had in D:OS 1 and reduced their power level. Give backstabbing to everyone, and there's no reason at all to play as a squishier fighter who does much worse damage.
(Updated with more elaboration.)
