Deeper Plasma integration
A feature that has been discussed is synchronization of settings, password and file metadata over cloud servers.
The fork of Nextcloud from ownCloud has brought about new questions regarding this topic: Will the Plasma team have to support two different systems? It would obviously be vastly preferable if both cloud implementations could be supported with one client, which would require collaboration between ownCloud and Nextcloud. The Plasma team has communicated this requirement for both parties. From an implementation point of view, at least Nextcloud features a key-value store which could be used to store client settings.
Another area with room for improvement is syncing tags, ratings and comments. Nextcloud offers these and would be able to sync them.
The most important part would be to write an Online Accounts module so you can setup Nextcloud to begin with.
What's next is, of course, to get to work. If you're interested in getting involved, let us know!