This is impressive. It seems like you really understand the requirements of the audience that uses spreadsheets AND data centric Python! The REPL, in particular, is a nice touch.
My biggest concern with a tool like this is that the power-iest power users who would love it tend to work at big enterprises where they're locked into productivity suite bundles with Microsoft or Google. Additionally, if you get traction, Google and Microsoft would be very likely to clone your features for Sheets and Excel.
My biggest concern with a tool like this is that the power-iest power users who would love it tend to work at big enterprises where they're locked into productivity suite bundles with Microsoft or Google. Additionally, if you get traction, Google and Microsoft would be very likely to clone your features for Sheets and Excel.