Well character editing is a unique feature to add to a game but I don’t see such a feature additaion being unique in design taking into consideration the easy of which technoligy convergence can take part of this and part of that and be combined to make a much more useful products .
If you take a cellphone and add a camera you have a product thats more useful than they would be on thier own and games have in the past included level editors in thier game years ago so the next logical step would be to included a feature that allows for character customization.
Would it be diffcult to do?
Well there is nothing about design at all thats easy so adding a feature like this would be just as hard as anything else as to design.
But
The trick here is not about code but about intergration in a manner thats transperent to the end user that fools them into thinking that they are still inside the game when in fact they are using two different applicaions that could just as easlly stand on their own.
Then again.
As an opinion, as I can’t see into the future, the direction of games developmet is becomming more about how quickly a game can scale beyound the ideals of the fixed design that through intergration or convergance the client will at least be able to see the game the way they want that goes far beyound the imagination of the design intent.