This series, I loved it. It was very enjoyable, however, I wish there was more to it and maybe like 24 or more episodes. Each episode has 2 scenes with each food menu name, 1 scene has one type of food and the other is the other food. I wish there was more though, I was kind of expecting a beach scene in this anime, or maybe we could get the Master Chef's name or something, it would be very nice if they went to the protagonist's world, though I'm not sure how that would end up.
The scene where his grandma gave him the key and to break it if he wants to end the cycle between the 2 worlds was very sinister. His face gave off an eerie vibe, it felt like he was actually going to destroy the key, but thankfully he didn't. Though, I am not sure what would happen if he did.
A hypothesis of mine would be, if he had destroyed the key, people would not be able to enter or exit the restaurant back to their own world and would be locked to the modern world.
Theory 2 would be, the door disappears and everyone inside fades away like dust.
Theory 3 would be, they all get pushed/pulled out of the time space [restaurant] and back to where they came from and would never be able to enter again.
Theory 4 is, they become trapped inside the time space and become lost souls that can't go anywhere.
Theory 5 would be, they remember what happened, but they forget the taste of the food that they ate, and the food they ate would disappear as if they had never ate it. Like, how some demonic forces in anime can reach into a time space and remove a body part from someone, like how Anos can take someone's source and crush it. They would also be pushed out and they would question if the Nekoya door ever existed or was just a dream.
Last theory would be, they would fade away into existence, they won't remember anything or who they are, and where they came from and people who knew them would forget them like a database on a computer file. Then a war would break out in the other world.
Though, these theories do seem absurd and ridiculous, it is fair to say that any of those may happen if the Master Chef had broken the key that connects both worlds.
I do wish they add a season 3 though, it was an enjoyable series. It felt like Food Wars, but without the ero cutscenes where the characters feel good inside from eating the food. It felt like Campfire Cooking in Another World, but without the Campfire in it.
I would also mark this as a spoiler, since the theories are, even though thought of and explained by me; the part about the key that connects both worlds is real and that might not be a good thing to include, however, if a theory has no source, how do you explain it?