Hmm... What about making it Not guaranteed?
As it seems, it could be that I take some +3 items and mix them together, do a Hasty Hand and GG, got my +3. What about giving it more challenge by making it Not Guaranteed, maybe with a higher chance of success, but you still need to boost up your Quality in order to do it better?
It could be guaranteed that your item is a +1, but not a +2 or +3.