But that’s the thing, it would be easier to make it as a game over making a new engine.
It’s just a case of, “What’s the premise and specifics?” So, for fun, let’s think about it.
Chub, Chomp, Chill is probably the closest thing to your Idea right now. Conceptually, it’ focuses mainly on weight gain and feeding in an enclosed environment. I’d say an open world like Animal Crossing’s or the Persona games would be the best fit for this kind of game.
Either a contained randomly generated open world with notable landmarks or a straight forward pre-made map that’s made up of multiple location. Some of those games’ mechanics could also be used as a good inspiration.
Like in Persona 4, a restaurant that serves a special dish only on rainy days. You need high stats to finish it, but attempting it raises three stats. Could pull a Yakuza and make a game about eating all the dishes a town has to offer. Have secret menu items or even hidden restaurants throughout the town.
Similarly, we could look at AC and how the villagers AI schedule allows them to live dynamically. A NPC system that tracks the characters’ actions and allows them to gain/lose weight depending on their actions on that day. Like if a character goes fishing, they could lose weight from failing to anything or gain a lot of weight the next day after reeling in a big catch.