Customizable transfigured gems

what if there was a way for players to custom craft skill gems to create their own set of distinct behaviors without the need for the devs to create new skills themselves? At that point they could focus on creating new behaviors and the amount of new possible skills would just multiply each time a new behavior got added
i realize the obvious of support gems doing this to some degree, but what i suggest is something more along the lines of transfigured gems however customizable in the way they modify the skills that they are versions of