It's hard to say what is or isn't underrated or underrepresented, so I'll just list out things I like, whether they be scenarios or ideas or what have you: Tall to giant-sized fat women (obviously), contagious weight gain, vacation or trip weight gain where a lady goes somewhere and comes back way heavier, women not-updating their profile photos for dating sites/social media/etc. after gaining weight and what that can entail, reality alteration weight gain (you've always been fat!), anything involving a woman being TOO big (outgrowing clothes, breaking furniture, getting stuck in doors...), women eating a TON of food and the absurd cost of said food, navel play/the belly being an erogenous zone, fat women getting into a bathtub/jacuzzi/etc. and taking up so much space water pours over the sides...

There's almost certainly more but those are some of the ones I can think of off the top of my head.