Sending to Graylog would be my personal use case.
I think reading from graylog would be a future thing that might be interesting as well I think, specially since many things can talk to graylog, but graylog also can do things like send alerts (use it as triggers).
As a side note in case you are not familiar with graylog: Below the surface it has an Elasticsearch backend, and uses pretty much most of the syntax from ES as well, so if you make an ES node, that would also be possible to share some code with querying Graylog as well.