>>11370699
>i dunno howd you fit weight gain into anything else
Off the top of my head:
>Strongfat
Your gf is training for a competition. Be supportive and help her bulk up. The game can be in keeping her in the right zone: more food makes her bigger, but at the end she still needs to compete, so some of that has to be turned into muscle with exercise. If you gain or lose too much, she can't compete in the weight category she wants. But maybe making her fail the competition isn't a game over, if you manage to convince her the weight looks good on her.
>Preg
Your gf is having your baby. Make sure she's eating for two. The game would be in pampering her to relieve stress. Or maybe hitting her random cravings like in the cooking/serving games. But having a failure state/game over will probably be too grim for most people. (Miscarriages happen more often than you think.)
>Royalty
The princess is to be married off, and the dowry is "her weight in gold." So, max that. The game might be in keeping her appealing to suitor(s), or at least mobile enough to walk down the isle. Alternatively, if we're talking islamic sultans, the game is in "how high will my daughter rank among the harem/wives" and the high score would be the suitor making her the first wife right away.
>Fantasy
If magic is powered by lifeforce, and bigger people can spend more of themselves without stopping/dying, the obvious choice is to overfeed magic users before rolling them into battle like towed artillery. The game might be in the management of a realm's defenses via fat magic girls, but also keeping in mind how much the realm can produce in order to keep them fat through potential combat engagements. Maybe you can conquer some land to feed your army, but maybe if someone sees that your mages are spent after a fight they'll attack you? Can you maintain fat parity with another kingdom, or will you develop a "fat gap" in your hot/cold (depending on what you serve) war?
And this isn't even my fetish.