It occurs to me that if possible, direct art replacement of original art assets might actually be preferred. Its much less fun, but if you implement a conditional WG system as defined by my example, you need to change the calls for that character’s emotion in pretty much every future scene.
So, forewarning to writers- the WG method I go into would actually, paradoxically, best be reserved for one-off encounters or people with few appearances. Since we have acts to work off of, I propose narrative gain for returning cast members over time at specific narrative moments, disconnected from player agency in any way (you can write around this, but don’t let us make a real meaningful choice here) to make this WAY more feasible and easy to code.
Not sure if the game is still split into three parts (its been years and I never got to pt.3 back in the day, gimme a break), but if so, you can replace base-level graphics with fatter sprites going forward, so by part three the main cast are the heaviest they can get (visually…), greatly simplifying workloads.
Let’s also get real. There are narrative exchanges with multiple ladies on screen. If they’re morbidly obese, they might not fit on the screen properly without significant overlap. Yes, that’s hot, but it’s also a potentially terrible looking art problem. We can almost certainly adjust where a person appears on the screen, but this should be something to take into account when creating character art; what’s the biggest dimensions we can go with, basically, before we suffer severe clipping issues. Adjusting character appearances with x/y coordinates would also greatly slow down work. Something to avoid. My recommendation might be to consider, at very fat sizes, making new poses or ‘zooming out’ to gain more space. The zoom out option is not ideal. This problem does not apply to many bosses that never interact with other people (on the same screen) - so in those case they can be as screen-encompassing and blobbish as you want.
A lore note on fat dynamics; the setting is a typical high-magic high-fantasy environment. We can handwave mobility issues and whatnot with magic, which is quite common and available. Y’know… in case you wanted to make Alice turn into the fattest Snek but narratively she needs to be able to like… adventure and have agency and stuff.
Alternative avenue if the whole ‘three game arch’ thing is undesirable: You could in theory add a new character, or recycle an old one ala the games many modded ‘side-stories’ (narrative mods) to replace the original story entirely. That is the easiest way to limit scope and develop content that can be releasable, but for fairly obvious reasons its not really super ideal. Audience demand is pretty clearly aimed at experiencing a sweeping epic featuring an all star heavyset cast. Thankfully, assuming the initial release laser-focuses on a small handful of characters, you can control the projects bloat. A new patch for each new major character rework, perhaps? Don’t know. I guess you could adopt an area based approach, focusing on any encounters in a biome and doing the whole thing bit by bit, that could work. Its likely that approach will take years to fully realize though- better I think to focus on a holistic experience with select party members, then growing it from there.