For a traditional office suite, you'd want a word processor with somewhat of a decent layout engine in it, a spreadsheet, something for slide presentations, something for rudimentary databases/datasources, and some sort of diagram drawing capability, and all of these integrated together.
By a "decent layout" engine, you'd like the ability to change fonts, add spacing between paragraphs, segment the page into regions, e.g. by changing the number of columns on a page, insert images and diagrams and choose how text wraps around them, create captions for embedded media, set page numbering policies (or navigational policies for web generation), generate table of contents, set table headers, make more complex tables with merged cells and different types of boundaries, generate table of references, generate tabs, then export to a web page, to a pdf, or to whatever other format you want, hopefully all from the same source. Then when you send it out for review, people can attach their comments to portions of the document, you can accept or reject their changes, there is a document revision history, etc.
So for example, you could write up a quarterly report by importing summary financial data into your spreadsheet, doing some basic analysis, export tables to your word processor, generate some bar charts or graphs, draw some boxes and arrows with your drawing program, stick that into your word processor, add some footnotes and hyperlinks, put the same info into a slide presentation. Then if you want, you could save that as a pdf or turn into a webpage, etc.
One time I had to write up documentation for various security certifications, and that was my introduction to the world of Microsoft office. Learning office made the project more fun, I had never used it before and was impressed with the functionality.
That said, I don't think most people who have office suites actually need a full powered office suite. Probably markdown + slack is enough for a lot of people, but I always like a good drawing or diagram.
Most people I know have shifted to google suite, it really has everything you need and is OS independent. But apart from the convenience of being browser-based, it is just another MS Office clone. The google drawing functionality is very basic, I often wish you could integrate something like excalidraw into it.
I am not suggesting this project needs to keep adding clones of MS Office functionality until it turns into Libre Office, but funded by the French government. That would be a waste. Instead, why not make it better? Reimagine it? Look, for example, at excalidraw. It's fantastic. It was really a fresh take on the stuff people were doing with visio.
I think there is lots of room to make a truly modern office suite instead of another MS Office rehash, so I would encourage this project to go its own way and do something interesting if they intend to be a replacement for proprietary office suites like google suite or MS Office.