+1. I support having a janusgraph-foundationdb repository under the main JanusGraph org on GitHub. There is precedent for this with janusgraph-docker, language-specific client bindings (janusgraph-dotnet and janusgraph-python), and janusgraph-ambari.
It comes down to whether there is a consensus desire to collaborate together instead of having separate forks. There would be some work from those fork authors to agree on how to merge those forks into the one that will serve as the initial repo. There would be more restrictions on getting code committed since the project would be expected to have a code review and approval process in place, like the other JanusGraph repos. Keep in mind that having a repo under the JanusGraph org doesn't mean that it needs to be production worthy from the start.
Thanks to both of you for getting the ball rolling here with FoundationDB. Happy to hear more feedback/discussion from the community.