Texinfo adds macros starting with '@'. I'm curious how to do that in plain TeX because I'm trying to create a simple TeX framework for my friends' needs and it would be more readable for them.
texinfo essentially replaces plain TeX's backslash with the at sign. This is done by setting the so-called \catcode of the at sign to 0.
Note that with this setting, @command means exactly the same as \command, you don't get a second family of command names.
Note also that designing and implementing a new TeX format is a lot of work; it is much easier to resort to one of the existing TeX formats.