Alright, let’s use this as a platform to jump off of. First, I think you should decide if you want weight gain to be the objective of the game (or a necessary byproduct) or an optional goal to pursue. If you’re specifically trying to make the customers fat, that’ll result in a different game than just trying to make bank. And if you are supposed to make money, what do you do with it? I can envision a gameplay loop where you make money, use that money to buy better stuff to meet customers’ needs, which gets you more cash, etc. Like you said, more spending = more gains, so this is probably most compatible with trying to get the characters fat.
Let’s talk characters: different customers will have different reactions to weight gain. Most will likely be averse to it, but a couple may be apathetic or even gainers themselves. What you say to customers will probably influence their dieting decisions, which affects their purchases. So, let’s say character A doesn’t wanna get fat. To keep her gaining, you’ll have to appeal to her sensitivities and personality by complimenting her and such. By contrast, character B might be easier to fatten up because she’s specifically trying to gain weight. Some may have a weight problem by default and start out fat/get fat up to a certain point no matter what; in which case, adding to their weight will probably piss them off, but you could also appeal to their self-esteem by telling them you like fat fucks.
Now let’s think about resources. Upgrades are the most straightforward way to meet customer demands and fatten them up, but you can make a more challenging game if they’re not permanent upgrades, but more expensive consumables you need to stock up on every few days. Buy better coffee/snacks/etc. and more specific stuff to this end, that kinda thing. It could also be interesting if you could spike a customer’s order with something to make them gain weight. Abuse this, and they’ll find out and have varied reactions to it; get their consent to do it, and they may even pay you extra for it.
If you want to add romance options, this can be tied to the gaining aspect. If a character wants to gain, you can help them; if they don’t, you have to make them okay with it or just give up in that department. Whether or not they let you help them gain, however, should probably depend on whether you can actually win them over on the idea, and may even be limited by whether or not you’ve romanced them. So, for example, character A needs to get to like you first, and then she’ll let you fatten her; character B will be the opposite and like you if you help her get fat.
I think it’d be good to hear what ideas you’ve already had and what you already like; this’ll give you something more concrete to start with.