Having collabora as a replacement for WebODF would be very useful. Currently, the WebODF module is built in a way, that the document is handled by FileController.php - not sure how collabora would know how two users are working on the same document.
If anyone wants to put his/her hands on:
Here is the documentation: https://www.collaboraoffice.com/code/
And here is a guide for nextcloud: https://nextcloud.com/collaboraonline/