Well, I like the idea but there is a reason why it could fail:
monsters can't be pickpocketed for items they wouldn't drop on death anyway
So basically if you have a habit of killing everything you would risk pickpocketing anyway there is no point of using the skill in most situations. Success of this RFE can be roughly measured this way: chance of getting a valueable item multiplied by percentage of monsters you would like to pickpocket. Monsters you wouldn't like to pickpocket are those who could spoil in-game progress (when a town or even a particular NPC turns hostile) and those who are not risky to kill.
In order to make these equations more successful I can propose:
- Monsters which are targets for pickpocketing but aren't target for killing should carry really valueable items.
- Some monsters must be guaranteed targets for pickpocketing: e.g. Terinyo folks are perfect candidates: they could turn hostile on kill but could forgive a pivkpocket eventually.
- Make chance for failure unlikely (gurantted for high-level thieves?). Make it increase with unsuccessful attempts over time (well, NPC would notice you hanging around for too much).
- Make it clear how well you're prepared for theft from this particular NPC in your current state in the given place. By either giving a word (impossible, unlikely, surely etc.) or a percentage.
- Add at least some situations outside of the town where pickpocketing is preffered over killing?