That is actually a really, really tough call, though I’ll do my best to answer.
I think a text based simulation would be the style I’d go for. While I’ve played and enjoyed a number of the fantasy combat rpg style games, they don’t really do it for me. Someone who weighs over 500 pounds going on an adventure, fighting monsters, and saving the world? That’s a little much.
On the other hand, continuing with the same fantasy setting, a young woman who moves to her Grandma’s village to help her run a bakery. And as time goes on, your character’s skills improve and she becomes more successful, but at the same time, she’s eating more and more. From a thin stick of a girl on arrival, she slowly balloons with fat until she starts having a lot of difficulty dealing with her added poundage. Eventually, she knows she should stop, but she just can’t control herself, and her weight continues to soar.
Another thing that does bother me about a lot of the fetish games is that getting fat is often a crippling detriment in the game. Simply put, you can get too fat to keep going. I found that with my character in No One’s RPG. Eventually, she got so fat that she used all her stamina to move a single square. While it was rather hot that she’d gotten so massive that she was useless for anything other then eating more and getting even fatter, it meant there was a lot in the game that I simply couldn’t do unless I held back. And let’s be fair, asking someone to hold back in a fetish game is just odd.
That said, I would certainly want some negatives to occur at higher weights. This will probably be best explained in another example. In this one, the player character is a young woman, either in college or on her own for the first time. Perhaps she’s become the guardian of her younger siblings after the untimely passing of her parents. Regardless of exact circumstances, you find yourself trying to make enough to keep yourself and any dependents going. The work scene doesn’t pan out well, but eventually you find the means to keep yourself afloat, either as a gaining camgirl or a personal feedee to someone with money. Naturally you start to gain weight, slowly at first, but rapidly picking up speed as your stomach stretches out and your daily consumption rises.
This is what I’m getting at, in a nutshell. As you get heavier, the core gameplay elements, in this case cash (and grades, if applicable), goes up and life gets easier. But the fatter you get, the more inconveniences you have to deal with. Outgrowing and bursting out of clothes, increasingly expensive purchases of new articles, a rising food bill (assuming you’re paying for it), decreasing mobility, etc. After certain thresholds, some activities become difficult if not out right impossible for you to do; household and yard maintenance, for example. So what do you do? Do you hire help? Does your feeder take care of these things for you? Or, in the context of being their guardian, do your younger siblings take over these duties, leaving you to be the primary bread winner for the family? Or in the college example, what do you do once you’re too fat to fit in the desks, or outgrow the doors? What do you do if you have a final being held on the 4th floor, the elevator is broken, and when you weighed yourself this morning, you were over 600 pounds? And then there’s dealing with the reaction of friends (and family, if applicable). It’s very unlikely there they’d be exactly thrilled with the fact you’ve gotten so fat so fast. Or maybe they’re totally okay with it. Maybe they’re a large (heh) part of the reason you’ve gotten this way. Either way, how do you approach it, or deal with it?
I probably rambled on a bit more then I should have, but the core idea is there. Simply put, I feel there’s far, far better ways to deal with the detriments of a fat character other then “you can no longer walk, thus lose the game.” Likewise, there should be some benefit to being fat. Players, myself included, don’t like playing games that punish us for enjoying the core features of the game.
And as you probably guessed from my examples, I really like playing a female feedee, especially a reluctant, bottom heavy one, with a, shall we say, generous bust line. I also enjoy games that let me be a male feeder, though given the option, I’ll generally choose the former before the latter.
And, as suggested by @Slime_Candy, a game where you’re subject to your feeder’s whims would be pretty baller. I’m not sure how it would work, but damn it, someone needs to figure out how.
Man, I’ve really got to learn how to program these things.