Thank you for reaching out and raising your concerns about the discrepancy between the in-app marketplace and the api.marketplace.mattermost. I will be sure to share this feedback with the rest of the team for you.
The Mattermost in-app marketplace is designed to display plugins that are officially supported and maintained by Mattermost Inc., to provide users with a curated, high-quality selection of plugins. This is why the number is currently limited compared to the full list on api.integrations.mattermost.com. I believe the team designed it this way to provide users with a more manageable selection and guarantee a certain level of quality and official support.
That said, I feel the demand for a wider array of plugins. Mattermosters who wish to install additional plugins not listed in the in-app marketplace can download them directly from the Mattermost Integrations Directory at https://api.integrations.mattermost.com and then upload the plugins to their Mattermost server via the System Console.
For the specific question about the Nextcloud plugin, this plugin is not directly maintained by Mattermost, and therefore, is not listed in the built-in marketplace. However, you can still install it manually. Please follow this guide on how to install plugins manually: Redirect
We appreciate your feedback, and it will definitely be taken into account as we continue to develop and improve our platform. If there are specific plugins you’d like to see included in the in-app marketplace, feel free to mention them and we can investigate the feasibility.
Don’t hesitate to reach out if you have further questions!