What kind of creature WOULD Asgore become if he did end up absorbing all seven souls? What kind of creature does Gaster think the King would become, if it's different?
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It’s really hard to say given what we know about soul absorption, haha. We only really have one example to go off of, and Flowey isn’t a good example because he’s specifically not a monster. He’s an empty vessel designed to hold souls. But that’s all we’ve got! So we can extrapolate a little based on what we see of Flowey, but it’s hard to know if something like that would happen to all monsters or just to him. And while Asriel did absorb a soul, we have no idea what he looked like afterwards, only how it felt to him. So really, what happens when a normal monster absorbs seven souls is anyone’s guess.
One theory I like is the idea that the form that monster takes when they absorb 7 (or maybe even just 1) soul reflects what that monster specifically associates with power, strength, control, maturity, that kind of thing. It’d explain why Asriel’s form draws so heavily from both his parents - when he has to imagine himself as powerful and adult, he thinks of them. So the monster’s body changes to become what they want to be, in a way. This is assuming that the monster’s form is controlled by the monster and isn’t a mutual decision between the joint souls, or just some random 50/50 combination of monster and human…
That said, I’m not really sure how Asgore’s form would change if he did absorb the souls. What would he associate with power, or the task at hand? What kind of being would he want to become? How would he want to present himself to others? Being an adult, I wonder if his self-image would be more controlled and reasonable than Asriel’s, but again, this is all total speculation. We really have no way of knowing.
I can see Asgore+7 as big, but that’s all I got, haha.
When Asgore and Gaster worry about what Asgore will become when he absorbs the souls, it’s in more than one way. There’s the physical sense of course, since no monster has ever absorbed seven souls, so they have no idea what he’ll look like when it happens. I think on their list of concerns about it, that’s low on the list though.
A bigger concern is how much control/awareness the souls he absorbed would have. We know from Asriel that he shared control with the body mostly equally with the kid, although Asriel still ended up with ultimate control. We also see with Flowey that he could control all six souls fairly easily, but the souls could also be convinced to rebel, and he lost control of his body. Although again, Flowey is an unusual edge case since he’s more like a container for the six souls. So maybe the six souls rebelling is something that could only happen to an empty vessel. Or maybe not. Again, it’s hard to say with how little we know about it.
Either way, living with seven human souls permanently sharing your body is going to have an enormous effect on who you are as a person, especially if all seven of them can also jockey for control of the main body. In an ideal scenario for Asgore, he’d still have 50% control, with the other 50% split between the seven souls, thus rendering them mostly unable to stop any choices he made, but who knows. Maybe it’d be split seven ways evenly, so he’d constantly be struggling. Maybe the seven human souls would overpower him and he’d lose control of his body completely. Either way, something like that is going to change who you are as a person, and both Gaster and Asgore are worried about that.
However, even assuming a scenario where the human souls were completely subsumed into Asgore’s consciousness or even all supported or agreed with his plan somehow, and Asgore found the willpower and drive to actually see his plan through (very unlikely), and he didn’t get killed in the process - even then, who Asgore currently is will be lost. You don’t personally wipe out an entire species planetwide and come out of it the same person you were when you came in. That’s going to mess you up. Both of them know that. Really, if Asgore absorbs the souls, then the destruction of his current self is pretty much guaranteed no matter what happens. Even if the whole plan goes off without a hitch and humans all die and monsters take over the world, Asgore won’t live to be there. Even if he lives, he dies.
A sad image that often comes to mind is Asgore+7 struggling to keep conscious and coherent with seven children screaming in his mind and fighting for his body, and Gaster tagging along with him to try and ground him. Constantly reminding him of who he is, what he’s doing, who he is to him, his identity, so he doesn’t get lost. Encouraging him to keep going, since if they’re paying this high a price for this opportunity, they can’t waste it. They have to ensure the world will belong to monsters and monsters alone. Asgore collapsing and starting to blur into nothingness and Gaster rattling off every memory they have together to try and keep some sliver of him intact… it’d be awful in a lot of ways, haha.
Edit - Oh dang, I actually forgot how Flowey+6 couldn't control the SOULs, but Flowey+7 could as far as we can tell, without them influencing his behavior at all, so possibly +7 is the threshold where a monster becames powerful enough to exert total control over the SOULs they've absorbed? Hard to say still, haha.
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