I think the idea's good, but I don't think you could modify Neovim to support it while retaining the ecosystem – not without also modifying much of the ecosystem, at which point it may be easier to do a straight port to a different editor, or construct a compatibility layer (essentially reimplementing enough of Neovim in the new editor that the ecosystem still works).

nmap can shadow the built-in commands, replacing them with whatever function you want.

> There some cases were Neovim already wants to disconnect itself from vim legacy like allowing to scroll viewport without moving the cursor

I think you mean beyond where the cursor is so it's hidden, but you can jump the screen so the cursor's current position is at the top/middle/bottom without moving the cursor using zt/zz/zb.