I know this is a bit long after the original post, however, I do think there is a fairly easy way to solve this. I believe that Pokemon Empyrean has a good way of tackling this, adding some sort of multiplier to experience gain via a better kind of lucky egg or exp. share. It would not solve the eventual problem of the exponential exp. growth, however, it could move the goal post forward a few more levels unless you want to have some sort of special Audino trainers that give extra exp. (Pokemon Empyrean does this too with the Mech Blisseys).
