There are no dedicated community repositories, everything is open and managed by Nextcloud GmBH. When github was acquired by Microsoft there was a longer thread here to move from grithub (a few independent apps use other repositories):
Unfortunately, the main developers and the company people are rarely on the forum to discuss this properly. You probably can reach them per Mail or at Hackathons or NC conferences in person.
Another problem could be that once the development is open, all your contributions will be visible and anyone could analyse them. Well, for a company that worries so much about your data and you can keep them safe and fully control them, it’s strange to have a different strategy for their development 
also a bit related: Why does Nextcloud use GitHub and not have a GitLab instance?