Remember the game-rec rules. You have to have done it or seen it done. Given that criterion, and assuming the question itself doesn't put it out of scope by saying "no really, something published" then a hack is... kinda OK as an answer, if it's a hack that's online that someone can use. "I did my own private hack" isn't an actionable answer. But if it's a public one, sure.
The reason I say that it's "kinda OK" is that there's a difference between most "hacks" the way the community uses the term and a total conversion. It's one thing if someone says "I took the Apocalypse World engine and have fan-published Aquarium World, a full game about being a fish." That's 100% OK. It's another when there's just a blog post saying "Oh in AW you could all be fishies, just replace a couple drives and flavor it, you're done..." Many such "hacks" have not even been used by their originator - it's the indie game scene wankery of choice, the game-design version of a "bad subjective" SE answer.
"I'm sure you could hack that" is of course totally off limits and is the main reason why we have the game-rec guidance, because everyone is sure their favorite game could totally do that... You can't suggest a universal rpg or hack or whatever because "it could work" - that violates Good Subjective, Bad Subjective.