If the new mode is similar to an old one, it is often unwise to modify the old one to serve two purposes, since it may become harder to use and maintain. Its definition is distinct from that of Text mode, but was derived from it. Using alternative major modes avoids this limitation. See Recursive Editing . You can look at these libraries to see how modes are written.
Text mode is perhaps the simplest major mode aside from Fundamental mode.
Rmail mode is a complicated and specialized mode. Major Mode Conventions Coding conventions for keymaps, etc. Example Major Modes Text mode and Lisp modes. How Emacs chooses the major mode automatically. Mode Help Finding out how to use a mode.