It does already exist[1]. Although I'm not particularly familiar with the "core" LibreOffice Online, the Collabora Online[2] branded version is super easy to get running on Nextcloud[3] (via Docker).

[1] https://www.libreoffice.org/download/libreoffice-online/

[2] https://www.collaboraoffice.com/libreoffice-from-collabora/

[3] https://nextcloud.com/collaboraonline/