Have you considered a hybrid solution, something that generates a contract from a large corpus of data, which can then be deployed statically?

I consider "responding to change" as a somewhat different scenario from "heterogeneous but not changing". So statically generating a contract from an existing corpus supports the latter.

I could also envision some kind of graceful degradation, where you have a static contract, but you have dynamic adjustments instead of outright failures if the data does not conform to that contract.