Alternative fee functions for the Lightning Network

Question

The current Lightning Network specification allows channel peers to specify a base fee and a proportional fee. In other words, the fee charged is a linear function of the forwarded amount. While that sounds completely reasonable, my question is, have there been any other proposed fee functions, perhaps to more closely align with node operator incentives, or to make path calculations easier? (Feel free to include the "zero base fee" proposal for completeness.)