I know I said earlier that I wasn’t going to add custom races, but I’ve decided to change my mind (sort of). My idea was to originally only limit it to demons and half demons from Luxuria Fantasia as a unique mechanic for them, but I’ve also thought of a few other places to insert it. I’m thinking of adding the ability to turn into/be turned into an imp (like in the WIP sketch below).
I was just making this drawing on my own, and was considering maybe using it for demon related events, but I figured there was quite a bit I could do with the concept. It wouldn’t be a race in the same way as the other LF ones since it’s more of a form than race (kind of like the dragons shape shifting abilities in LF). The reason I would add it to a weight gain related mod is mostly due to how much potential weight gain related content can be stapled onto it due to their smaller size and particular body shape. I’m thinking of making the form available to the races; Demon, Half Demon, Fallen Angel, Human, Elf, and Orc.
So far the basics I’ve got for the form are a huge penalty to personal combat skill, -3 martial, +5 Intrigue, -10 opinion from characters following Abrahamic religions, and +20 opinion of Demons.
For the base weight gain affects, you will gain extra weight from your diets. So it will be easier to gain weight in this form, and harder to lose it. If you are below 20 weight (pot belly tier), you will gain a little additional weight every weight pulse until you are at least at 20. During dinner dates, the cost will be discounted if you or your partner is in this form (even larger discount if both). You will be given a small amount of money every time you choose a new diet for yourself (because of the smaller size, the character wouldn’t need to eat as much. Those with the Reaper’s Due dlc will also get more money based on how prosperous the county you are in is). It is not mutually exclusive with temporary obesity, and will increase the amount of turns needed before the events detailing the larger sizes will fire. Females will have their current body shape flag remembered before turning (if you ever decide to go back to normal), and then you randomly assigned a different body shape (bottom heavy being the most likely, and no chance of it being breasts, use a potion if this is really desired).
There will, of course, be flavor events detailing the character adjusting to the new body (positive if the change was consensual, most likely unwanted if the character was cursed), the ease of falling into gluttony, the church getting upset (mildly, unless your character gives them a reason to be more aggressive) at your new form, some unique events for the different races, dealing with a larger pregnancy if your male partner isn’t an imp etc.
Now for how to get the form in the first place. Demons will be able to freely change back and forth whenever they wish. Half Demons can do it too, but they will have a chance of cursing themselves by accident and will need to lift it before they can turn back. Fallen angels can transform, but if they do they will be cursed 100% of the time, if they lift the curse though, they will lose the fallen angel status and get the demon trait. The rest of the races will need to be cursed to become an Imp, with one exception. If a character is born as a compatible race, and their mother was in imp form when they were born, the child upon reaching adulthood will be able to switch back and forth like the demon characters.
So how does one get cursed the normal way? First will be in a random encounter while hunting that I’m going to add. Probably going to be an event for the abandoned cabin. Second will be very rare encounter when going to the potion shop and leaving without buying anything (will still be player choice on this occasion). Third will be a decision for characters with the possessed trait, you can choose to expel the demon from yourself in a very crude ritual (remove the possessed trait), but you will be left cursed. Fourth will be if you own the monks and mystics dlc and a character in an evil cult of rank 3 or 4 decides to use their points on cursing someone (cult members can screw it up and curse themselves, or intentionally curse themselves too).
So how do you lift the curse? I’m thinking of adding another option when visiting the potion shop while as an imp, and try to get help from them. It’ll be free but have a low chance of success and a low potential for side effects. The other I’m thinking of is getting assistance from your religious head, a mystic, or a very capable realm priest. They will cost a lot of money (especially the religious head), but it will always work (if you are a fallen angel, the realm priest and head of faith won’t help if they are of an Abrahamic religion, but the mystic will if they aren’t either of those two things).
This is kind of a random idea I had, and I intend to flesh it out a bit more on the weight gain side before adding it, but I wanted to get the thoughts committed to text so I could look back at what I need to do. All of this will require that you have the full magic content turned on in the fatocracy settings, for those that don’t want it. Also as a side question, how is the sketch so far?
. Although the face reminds me a bit of Tristana from Legue of legends but I think this is because right now I’m tired of working and my vision gets a little strange when I’m like this. 
, apart from it’s your mod, it’s something we all have to respect. 
. The imp wasn’t based on anything specifically, but I may have made the markings on body a bit too similar to Midna without thinking.