I've never considered the equivalent for an RPG, but I've been boring folk for eons about my desire – which could only really be delivered effectively by a whole new generation of gaming, I reckon – for a new 'Vanilla' game franchise, ideally headed up centrally by the console makers, ie Microsoft. Customisation is far and away one of the most important things in gaming to me, and if they marketed a completely basic game package for the relevant genres – beat-'em-up, platformer, kart/racing game etc – with all the physics and controls and so on there, and solid as fuck, but left it to you to design the game however you want, I'd be over the frigging moon. So there's a dull but serviceable vanilla starting point, then you get to create characters, arenas, music etc, from a huge selection of options and variables, including using Kinect or the equivalent to put yourself and your friends in the game. The company then makes a further fortune by giving you endless different add-on packs to download, full of both original designs and great licenses, so you could make your own Muppet Kart game or whatever, or even have Kermit racing against James Bond against your next door neighbour, on a carefully fan-recreated classic Mario Kart track, and so on. Plus you'd have other online users checking your creations out, sharing their own, etc. That should be the future of gaming, for me. I want to be in control.
This could work for RPGs as well, like a 21st Century Theme Park – but that would be a lot of work. I suppose that's partly the appeal.