Consistency-Based Service Level Agreements for Cloud Storage (2013)[pdf]

The idea here is that instead of specifying consistency up-front (weak, eventual, strong, etc), sometimes users care more about availability than consistency, so the system lets users say "I want a response in 100ms, gimme the best consistency you have in that time".

There's also a complicated system of a more explicit tradeoff between consistency and availability in the paper, which I think is super interesting because it makes the tradeoff between the two apparent and easily changeable.