I think the time spent on trying to reinvent mail, could be spent on making a good web maps app.
Mobile already has good apps like Organic maps, so it makes sense to build off that.
But on web there isn't actually a great google maps alternative. It's going to require A LOT of work, but I think that area makes more sense than mail.
With mail, you either go the proton/Tuta route: end to end encrypted. Which requires a lot of expertise in cryptography unless you're copying an existing thing (at which point... what's the point).
Or you go the Fastmail/Startmail route and just have a regular mail service Butt then you have to worry about storing a lot of potentially VERY SENSITIVE data... and oh boy the day that stuff leaks.....