you can already be rewarded for having a full node. just turn your full node into a p2pool node with a moderate mining fee.
There's only so much room for p2pools. If all 7000 (or however many there are) fullnodes currently in existence ran p2pools, only a few of them would have miners on them (and those would be the ones with the lowest fees, so they wouldn't even make too much money off of it).
The only proof-of-fullnode is mined block. If you are not producing blocks - you are not bitcoin network supporter.
That's not proof of fullnode, only proof of hash power. You can mine without being a full node as long as you have a full copy of the blockchain.