My hot take on a better excel: the two things excel sucks at: version control and syncing. On the backend, separate the data and the logic in the spreadsheet and put each into version control. Then use something like syncthing to share documents with colleagues. You might also need something like bitmessage for initial handshake. Now you have a spreadsheet you can collaborate on in real time over the Internet or LAN without screwing around with a server, a google account, a credit card etc.
There's two more things excel is horrible at: choice of extension language and being able to graduate your spreadsheet into a real program. You fix the extension language by using something like web assembly on the back end, and probably bundle one or more compilers to go from $lang to web assembly in order to be user friendly. Lastly you fix the last problem by virtue of doing all of the above. The second two features won't draw in new users much, so they're less important in the short run but make it a lot more sticky.
I'm not in a place in life to put much free time into that project, and ideas are cheap ...
