While this may not be exactly answering the posed question, I think a latency based payment application could be very interesting. Quai’s network topology incentivizes miners to geographically organize themselves to reduce network latency. This could allow a multi-chain payment app to select a subset of a user’s funds in the optimal chain location to minimize payment latency/time based on a user’s geographical distance from a node/miner cluster.
Don’t think this could be done on a monolithic chain that currently exists. Feel to correct any mistaken assumptions I’ve made here.