You know, I've read a ton of 'Jujutsu Kaisen' fic, and I feel like a lot of writers kind of box Megumi into being either Yuji's stoic protector or a distressed damsel after Shibuya. The interesting trend, though, is the focus on his guilt. It's not just survivor's guilt from >!Sukuna's rampage!<, but this deeper thing about failing as a jujutsu sorcerer and a friend. Some authors really nail the internal conflict—he's trying to be a 'proper' sorcerer like Gojo wanted, but his personal loyalty to Yuji completely shatters that cold framework.
I'm less convinced by the fics that have him do a full 180 into being super emotionally open overnight. His development feels more like a slow thaw, you know? The best ones I've seen have him communicating through actions, not words. Like, small rituals. Making sure Yuji eats, or silently taking the watch on a mission so he can sleep. That feels more true to his character than big declarations.
Also, weirdly specific, but I've noticed a bunch of post-canon fics exploring his relationship with his own shadows after everything, and having Yuji be the anchor that pulls him back from getting lost in them. That's a cool angle I haven't seen much in the manga itself.