Sure. This "counter" can even decline by time. Lots of stuff is possible, the less fancy and the more "computable" the features are you have in mind, the higher the probability that one can implement it.
One of the most basic requirements I'd name for a pickpocket system is that the victim has to cooperate or at least be given the opportunity not to cooperate in an out of character way.
The PO can decide wether his char is a possible victim or not, regardless of the result of the theft attempt.
That was why I said one should only be able to steal coins. It is not a good system, but it could make being a thief a little more fun i think.
The sneaking thing wouldnt have to be so fancy. Just give a sneaking char some mark or somthing indicating that he is sneaking. At least that is better than how it works now. If somone sneaking manages to attack from behind he does triple damage or somthing. The problem would be making npc's act correctly I supose:/ Till somthing better is posible just let npcs ignore it.
Oh, and would be nice with an assassins atribute package.
I think there shouldn' t be a thief system, where cahrs could stole things without any RP. It would be really dissapointing to notice, that all the money you had was stolen. That would be only fun for the thieves, but no fun at all for their victims. And if my char`s money is stolen, I want a bit RP, so it can be fun for me, too. So I think a thief system would make really no sense, because many players would leave the game and much more would play thieves, and I think Illa doesn`t need that..
The PO can decide wether his char is a possible victim or not, regardless of the result of the theft attempt.