So we had a Documents app based on WebODF but it hasn’t been developed further for some time.
Then there’s also the joint effort with Collabora to bring LibreOffice Online to ownCloud, and hence this also works as an app for Nextcloud: https://owncloud.org/blog/libreoffice-online-has-arrived-in-owncloud/
@jospoortvliet any other future plans? If so comment here too. 