We are already supporting .test, .example, .invalid, .localhost, .internal and .local that are pretty much covering most needs.
There must be some kind of rule, otherwise we risk that any kind of proposal will force us to add even more suffixes. So far that rule has been that we'll pick RFC defined suffixes.
I'm not sure we want to go much further with the hardcoded list (unless this RFC proposal one day gets approved, but it looks like it elapsed), especially with it containing suffixes named after companies. We could maybe accept some names, if there would be updated statistics showing they are largely in use. Without clear evidence we could just make good or bad guesses.
I think that, at a certain point, we should instead improve our "did you mean to go to" notification bar to happen not just when I type a domain but also when I type a path after it (I think there's a bug on file, but maybe I'm misremembering cause I can't find it), and implement Bug 1635062, so it's easier to add custom lists of suffixes through an add-on.