I like that rewording, but I still think this should be a guideline.
There are certain mappers that have made an exclusive name for their guest difficulties. Examples are Blue Dragon (Nogard), kingsal2 (Alarmable) and I guess we can add Rin and even Kirby in here.
I think that, as long as it makes sense or it's related in any way to the song/theme or difficulty (like calling an Insane [Armageddon]) AND it's not way too ridiculous, it should be okay. I personally see no harm on having a Pokemon themed beatmap with difficulties called [Pichu] [Pikachu] [Raichu] or similar (or even beatmania songs with [Hyper] and [Another]), but I really think things like
this shouldn't be allowed at all, because they make zero sense.
(I'd also like to say that any kind of silly/annoying looking diff name such as [I~n~s~a~n~e] shouldn't be allowed either).