Kannik
Legend
Might Rule of Cool simply be a particular variation of/take on Rule 0? In this case tilted towards the favour of what's narratively exciting for the game/campaign at hand and/or what's the most interesting or fun for the players? Just like Rule 0, it's a philosophy -- Rule 0 could be taken to the extreme where the DM can simply reply "Nope, you still fail!" dozens of times in a row because they want to (could be because they think its more cool that way!), but for many of us (I'd assume) we wouldn't play at that table for very long if we didn't expect that going in. But I also doubt that many would be clamouring to get rid of that concept or saying that due to Rule 0 being a thing that it's ruined every game that's used it. It, like many things in RPGs, is middle path. Walk that (still broad) middle path and it can work wonders. Walk to the sides on either extreme, and it can be deleterious.
Separately, I've found that Rule of Cool can apply to the DM/GM side of things in terms of worldbuilding, both in terms of what they create beforehand or when creating a detail that wasn't preordained. There are a lot of options that make sense... which of them makes it more interesting/intriguing/exciting/fraught for the party and/or the players for this campaign? Lean towards that one. (This can work well when a player asks a question about a particular thing, such as the dragon seduction example above, or when the GM had something in mind but this question opens up new avenues and therefore would be 'cooler'.)
Separately, I've found that Rule of Cool can apply to the DM/GM side of things in terms of worldbuilding, both in terms of what they create beforehand or when creating a detail that wasn't preordained. There are a lot of options that make sense... which of them makes it more interesting/intriguing/exciting/fraught for the party and/or the players for this campaign? Lean towards that one. (This can work well when a player asks a question about a particular thing, such as the dragon seduction example above, or when the GM had something in mind but this question opens up new avenues and therefore would be 'cooler'.)