There's a lot of things I've had to think over for my upcoming post-Promised Day Pride fanfiction (which still needs a title). I haven't necessarily run into any problems like this in any of my Pokemon fics, so this is kind of new for me. It's a good thing I've been bringing a writing journal with me everywhere so I can jot down my thoughts. And since the Random Blog is in desperate need of some new entries, I'm going to transcribe a portion of the journal (nothing plot-related, of course!) here.

Another thing I've been thinking about is the people Pride devoured and the abilities he gained subsequently. What happened to them after the Promised Day? Are they still part of him? Selim retaining Gluttony's hunger and acute sense of smell would make for an interesting twist, but that would also mean Kimblee's soul still lives on inside of him. That would be quite annoying, considering the fact that I've been mulling over having Selim and Pride be two separate beings. Writing about three people conflicting within the same body could get pretty confusing. But since Selim is in fact a homunculus powered by a Philosopher's Stone imbued with souls, there wouldn't be a logical explanation for Gluttony and Kimblee's absence. Canonically, Selim probably was stripped of these souls, but then he would've also been stripped of his homunculus identity and obviously that won't work with my story.
This is a rather silly concern that won't even be mentioned in this story, but it's still odd to think about and if I were to handle this story realistically, it could create some major issues-- how does Selim go to the bathroom? Since I'm following canon, the shadows are Pride's actual form, and the Selim body is merely his container. It's impossibly hard to imagine the shadows taking a crap or whatever, since Pride is just a shapeless black mass. But he obviously does eat, and that stuff has to go somewhere... Even if it somehow isn't necessary for him to go to the bathroom, some pretty large issues are still present. Wouldn't he get suspicious if he didn't need to use that bathroom and every other human did? The only way around this would be if Selim was so sheltered that he was unaware of the fact that people went to the bathroom. But he does live with his "mother", though, and she obviously goes to the bathroom. So really, the only logical explanation that wouldn't create plot conflicts would be that the shadows are perfectly capable of going to the bathroom, even though I can't possibly imagine that. This depiction actually interferes with canon, now that I think of it-- Gluttony is a homunculus as well, and he is incapable of going to the bathroom from how I see it. After all, everything he eats ends up in the false Portal of Truth. From what I remember, none of the other homunculi are ever shown eating, except for the human-based homunculi. Since they have perfectly human structures, it's understandable that they could go to the bathroom the way a normal human does. However, the original Greed alludes to the fact that he may have to use the bathroom when he describes the advantages of Al's "immortality". It's likely that he was simply speaking from Al's perspective, though. I don't recall Envy ever eating anything, but I guess he'd be able to go to the bathroom-- in order to function properly, he needs a human host, which would allow him to use the bathroom like a normal human. Whether or not his true form goes to the bathroom is another question entirely. Father, whose anatomy is the closest to Pride's, was shown ingesting a melted down Philosopher's Stone, but this probably went directly to his bloodstream (if he even has one... I guess I could say it was absorbed by his liquid body?). On that note, Pride never actually ate Gluttony's body-- his shadows merely leeched from his Philosopher's Stone. He did appear to fully consume Kimblee, however. Maybe the false Gateway became part of Pride after he gained Gluttony's abilities. That would actually make a lot of sense-- but this brings me back to the thing about Selim being suspicious about not going to the bathroom. No matter which option I hypothetically go with, there's going to be some kind of problem.
I'm quite curious about Pride's anatomy in general. Like Father, Pride exists in a "leather bag" of sorts. I suppose it's an extremely convincing covering composed of the materials that make up a human's skin and hair and so forth. One episode featured part of Pride's face stripping away, and a shadowy extension sprouted in its place. This extension boasted an eye where its covering's eye would've been, and a mouth placed in accordance with its vessel's mouth.This seems to point out that Pride's true form possesses facial features beneath the respective features of his shell. One must wonder what Pride's nose looks like, as he obviously has a very keen sense of smell, and the only features of his that are actually shown are eyes and mouths (as well as arm-like tendrils). If his shadows secretly possess (a) nose(s), then perhaps they also secretly possess things that would clarify my bathroom concerns... Can Pride decide where and if his facial features appear? I'd assume he can, and I'd also imagine that his body within the shell possesses features in accordance with the features of his vessel and nothing more. Also, can he choose which eyes he sees out of during any moment in time? It would be weird and confusing if he saw out of all his eyes simultaneously. The manga actually made Pride's container appear lifeless when he revealed himself to Hawkeye (this was ignored in Brotherhood, though). This supports that theory nicely. Pride's transition from seeing out of his main body's eyes to seeing out of a separate shadow's eyes probably feels like a weird out of body experience. My guess would be that Pride cannot bleed and does not possess blood. Heinkel was shown slashing at his container, and it did not take physical damage (his clothes were torn, however). But later in the series, as mentioned earlier, Pride's shell started to fall apart. He was incapable of regenerating the vessel, too. His shadows have been shown to take damage from the intense light produced by flash bombs, although he seemed to be able to regenerate them, but not in the same fashion as the other homunculi. Actually, it's likely that he didn't take damage at all-- instead, the shadows were temporarily expelled or driven off. What kind of internal organs does Pride have? I'm guessing his Philosopher's Stone takes the place of his heart, and it's probably located where his container's heart would be. But what of his digestive and respiratory systems, and his brain and all that stuff? Does he not have these, or are they also located where his container's organs would've been? It's more likely that he's in a situation like Al's-- just a soul (Philosopher's Stone) in a body. But Pride eats stuff, so he probably has some kind of stomach (or, like I mentioned earlier, Gluttony's false Portal of Truth). Perhaps he simply absorbs the things he eats into his body, like when Father sort of swallowed up Hohenheim. Pride might be made out of a gas-like substance that lacks both shape and volume, and that's why he can take on so many forms and sizes. Pride's composition is difficult to think about logically-- he can take on any number of shapes and sizes, and yet he can easily tear through things like a knife. It's a good thing I don't necessarily need to apply logic to this story, since it's purely fantasy. I'd like to at least attempt to explain all these crazy elements, though. Lastly, what on Earth is that marking on the forehead of his post-Promised Day incarnation supposed to symbolize? Also, why didn't/doesn't he appear to have an ouroboros tattoo? Was it in some remote location, like under his hair or clothes, (or as Jo speculates, on his butt) or does he just not have one? It'd make sense for him to hide it, since he was masquerading as a normal human child. Is the post-Promised Day marking somehow related to his ouroboros? He obviously didn't possess an ouroboros in that location while he was Pride, so I'm not too sure about that. In a manga omake, it was explained that the marking was a button that would cause Selim to become Pride-- but that clearly shouldn't be taken seriously.