1. When sneaking, poisoned weapons should have all their normally "damage over time" effects be front-loaded. That way, sneaky poison users won't have to wait for the full effect to kick in and risk being detected by someone else. (Maybe this could be a perk).
2. Allow the player to see when an enemy is wielding a poisoned weapon. Also, increase the scope and power of poisoned weapons (slower move speed, hallucinations, bleeding out). If this is too cheesy for you maybe make this a perk as well. This is tied directly with number 3.
3. When blocking a poisoned weapon with another weapon or shield, have the poison not take effect until the poisoned weapon hits flesh. This would work for both the Dovahkiin and enemies. Shields would be able to block poisoned arrows while other shield-less characters would just have to try an avoid being hit. This would add a little more tension to fights with multiple baddies where you're trying to kill the poison guy first without being hit.
4. Allow for diminishing returns. Instead of having poison completely removed after hitting one guy. Make it last until it is wiped completely off. For every subsequent guy you hit, you'll do less and less poison damage.
I apologize if any of this has been suggested before, these were the most obvious changes I could think of.