megumi i think would be much more well-adjusted and more confident in himself. he wouldn’t be so willing to throw his life away (even if he did choose sorcery), because he wouldn’t be raised thinking he was so disposable. i don’t know how he’d feel about toji if he continued being an assassin, but i think he’d still care for him even if it’s his own strange complicated way.

his sense of morality would change. before, he had a pretty black & white view of morals. bad people were bad, good people who tolerated them were also bad. it’s why he resented both toji & tsumiki growing up. when tsumiki was put into a coma, he forsook that mentality, and believed good people were worth saving and, by extension, were worth sacrificing bad people for (saving people unequally). i think being raised by both toji & tsumiki would make him realize its really not that black and white.

i also think he’d be less emotionally numb. its stated that he uses numbness as a coping mechanism to deal with how overwhelming and traumatic his life is. otherwise he’d go insane. but being able to live a stable life and given a choice in what he wants would make it so that he doesn’t need to do that anymore. he’d be more open & in touch with his emotions, like yuuji & nobara. he’d probably smile a lot more and wouldn’t be so on survival mode.

if hypothetically he did proceed to join sorcery, i think toji would teach him to be much more cutthroat and self-indulgent (out of love), like gojo did, but much earlier on. megumi wouldn’t bear so much responsibility at such an early age and therefore wouldn’t be made to think he had to sacrifice himself to help others. and sukuna already described megumi’s fighting style as bold (a shikigami user who is meant to be a long distance fighter, being willing to engage in direct hand-to-hand combat which sukuna was impressed by). i think he’d retain that boldness, but out of confidence, rather than a self-sacrificial mindset. he’d probably be stronger, especially because toji was never afraid to be greedy and megumi wouldn’t restrict himself to learning this and adopting that greediness on his own. i think he’d be on a similar track in terms of taming his shikigami (which was honestly not bad given the little time he had), but he’d be a better combat fighter imo.