Stibium is desktop enviroment based on cloud storage and designed to run inside a web browser. It was built as an Ubuntu One app, but it could work with NextCloud

Stibium is a web application that provides an interface similar to a traditional desktop environment (it imitates KDE) but uses a cloud service to store your files. It can be seen as a desktop environment to manage your files on the cloud.

Stibium is powered by PHP, SVG, and Javascript, so it’s all Free Open Source Software.
We use the Ubuntu Font (Ubuntu Fonts) because is a free font, and it’s very nice.

Source code: https://invent.kde.org/unmaintained/stibium

Demo: https://www.zorbaproject.org/esperimenti/stibium/