I think Lillard's ceiling will be less about numerical averages, and more about doing the right things at the right times. The numbers will come as needed, but he's not selfish in that way. The nice thing is that he's already got a very high floor in terms of making the right plays when it's time to step up.
I don't know if Lillard has a ceiling, but if he does, I'd be willing to bet that he can bank a 3 pointer off of it and put it through the hoop with 0.9 seconds for the win.