I want to bring up this here as well.
We would like to have nextcloud as an backend storage for mattermost,
or a way to share files over the nextcloud with libreonline would be the killer app.
The original post can be found here on the owncloud forums.
I would love to see this integration between these 2 projects.
Hey @nev, I also now opened this issue at our tracker: https://github.com/nextcloud/server/issues/3202
Are you interested in getting this started, or know anyone who would like to? Integration here would really be very nice.