My firs topic here was very similar to this.
I don’t think it should require a specific profession to design new things in this way. It could be a stand alone modding program or a designer that you access from the main menu of the game. It would however be really nice if you could access it from within the game, so that you could tell one of your crafters to make this new thing that you thought of, without leaving the game.
Appeal, price, stats, required level of crafter and so on could be handled by an algorithm to keep it balanced. If you manually make part of the stats better then others will have to be made worse, or it will require a better crafter and so on. A super item will require a super crafter.
Still love the thought of this. It would be even better in multiplayer where you could have your own design and sell unique items to other players.