Just give them some sort of backstab bonus, if possible. Has been suggested before and should really be implemented.
Yeah, harder to implement, but much more balanced.
In case of implementing backstab bonus, forget about new stats. Just give backstabbing with daggers +100% to damage.
25 pierce * 200% = 50 pierce
Enough to kill archer. Now if you combine this with correct speed bonus, it can kill 70% of characters in cRPG in one hit.
You say: overpowered? Nope, it isn't. Any aware guy can quickly change the direction he's looking at, so that he doesn't receive the additional 1
00% damage and can easily kill the daggerman. Also, mind you, that backstabbing requires a stealthy approach and is
very risky.
I want such a mechanic, for sake of assassin roleplaying.
To balance things out, though, give daggers flag so they can't be used with shield and combine it with flag so they can't be used on horseback - I know that daggers are too short to use them on horseback, but if someone actually managed to aim it with
godly accuracy...
Oh, and if you want, you can also put flag "unsheathable" - I don't care. Just implement backstabbing mechanism for daggers!