The tendency so far is that you have to go up in rank in order to unlock challenges and Nightmare mode, so basically you have to go up in rank to make the game HARDER in interesting ways; adding cheats would of course make the game EASIER. But I think it would be important to make the game easier in interesting ways, too. If you look at the mortems posted in the forum for mortems, some challenges are more popular than others; there are more mortems for Ao100 than for Angel of Darkness, for instance, even though Ao100 is harder than Angel of Darkness, and I think this is because Ao100 is more fun than Angel of Darkness. What I'm saying is that cheats would need to make the game easier in fun and interesting ways, and not all of the suggestions on your list of cheat modes look to me like they would change the way the game plays in an interesting way (the suggestion for AoD would completely nullify the effect of the AoD challenge, for instance; and Ao100 already has no Cyberdemon or JC). Something like changing each monster in the game to some number of former humans based on the strength of the original monster (maybe change an imp to two former humans, for instance, and a hell knight to seven former humans, or something) would make the game easier but in a way that changes the style of the game in a way that might be interesting, at least for a couple tries at it--you would find the rocket launcher unusually useful.
Anyway, you have to unlock Nightmare by gaining ranks; along the lines of the unlockable cheats suggestion, it might be amusing to have a skill level EASIER than the other available ones (instead of HARDER, as is the case with Nightmare) which is unlocked by getting a very high rank. Something insultingly easy might be a good joke, like having only one monster per level.