IDEA:

- Full nodes advertise a bitcoin address.   Users that need to download the
block chain from that node can be encouraged to send a tip to the peers
that served them (by % served).   Recommended tip of 10mbit should be fine.

- A full nodes can *require* a tip to download the blockchain.  If they do,
users that don't specify a tip cannot use them.

CONS:

For some people, this may represent a barrier to hosting their own full
node.   After all, if you have to pay $15 just to get a copy of the
blockchain, that just adds to the already expensive prospect of hosting a
full node.

PROS:

As long as you manage to stay online, you should get your money back and
more.   This is the an incentive for quality, long term hosting.

In the long term, this should cause stable nodes to stick around longer.
It also discourages "installation spam" attacks on the network.

Fees for other node operations can be considered if this is successful.