Hi Andreas,
as you may have noted,
https://github.com/arangodb/elasticsearch-river-arangodb is a java implemented adapter that is a user contribution by Patrick Huber.
However, upstream ceased support for the river interface, the Logstash API only supports ruby adaptors.
In the short run there won't be resources in the ArangoDB core team to implement this.
We would however (as we always do) appreciate and support community contributions.
Cheers,
Willi