A protocol level change to incentivize nodes would be very difficult, if even possible, for so many reasons.  Anything with incentive will be hit hard by scam attempts.  Also, consensus for changes becomes more difficult as bitcoin project grows.  You can be sure that any BIP to incentivize full nodes would be debated as much as changing block size limits has been.  I don't see it happening, personally.

FWIW, the bitnodes incentive program is now going to be continued past the original expiration date, thanks to 21.