> why not fork LibreOffice [...] But after some thought, I feel a cloud collaboration suite makes more sense

LibreOffice has a cloud version :). From what they presented at T-Dose like 10 years ago, it's basically an instance of the software running on the server, cut up into tiles and displayed on a webpage as zoomable image using Leafletjs, the same way that google maps worked before switching to vector graphics 15 years ago. Clicks and other input events are presumably emulated on the server and the resulting display update is sent back to the client, a bit like VNC but using a map library