Great solution but that'd be a pain for anyone using an app which supports (theoretically) 3D Touch and the pencil cause then you'd have to switch between inputs I.e from pencil to fingers just to get to the feature you want which results in a substandard user experience. Albeit it is a trade off worth it for both technologies.
Since the Pencil has its own pressure sensors, you would have to separate the inputs anyway. Presumably, you'd have the option to make them do the same thing if it makes sense.
As for probability, I think that next year is iffy. 3D Touch added so much weight and thickness that Apple would have to have a *major* savings lined up in order to do it on an iPad scale. I could easily be wrong, but I'd put a higher probability on two years.