I have no preference. My use cases would always be pretty simple of just a crop and a format change which would be similar enough across any URL format.
It may be worth considering doing something thumbor compatible, as it seems to the most established URL format for this sort of thing. A lot of “client” libraries across languages Libraries — Thumbor 7.8.0 documentation . Even Elixir has one.
It would be cool if the “client library” (the URL generator and signer) uses more or less the same piping style that Image itself uses.
However, I do not have much experience in this field. It may be that there exists better URL formats that are more flexible or easier to parse or whatever. 
