Overall, I think the three main things that needed to be done are:


\##Tsumiki Fushiguro


*Tsumiki as a character needed to actively show us why she's Megumi's moral anchor. Giving her more of visible presence in the early arcs would've given Megumi’s arc more weight.*


Tsumiki should've started out as more of an active character in the early arcs. Whether that be through packing bento boxes for him or scolding bout his injuries, just anything to establish contrast between Megumi’s Jujutsu world and the mundane world he wants to protect. While we're at it give Tsumiki on-screen interactions between Yuji and Nobara, which could maybe have her embarrass Megumi by treating him like a kid. 


This could also give credence to Megumi's philosophy on heroism, which is a classic case of telling us this philosophy, but not truly showing the foundations. Have their be flashbacks detailing how they handled Toji's abandonment. The philosophy foundations could best be shown in an argument between the two, highlighting the contrast between Megumi's cynicism and Tsumiki's idealism. Have Megumi grapple between the two halves constantly.


When Tsumiki finally awakens for the Culling Games, having her be immediatly possession by Yorozu ruined any chance for her to be seen as anything more then a prop. Have her actually wake up as herself first for a poignant reunion where she can actively see how much he has grown and express horror at the dangerous world he entered to save her from. When Yorozu tries to take over, Tsumiki’s soul should actively fighting back from within in the same vein as Yuji and Sukuna.


\##Ten Shadows Technique 


*His taming over the Ten Shadows technique should've been more about actively unitilizing the shadows and focusing on fully completing Shadow Chimera Garden, rather than just being the precursors to MAHORAGA.*


Megumi needed to conquer every untamed Shikigami, complete his DE, and merge each shikigami ability over the course of every arc he's in to give him a sense of earned progression instead of giving all that to Sukuna. Dedicate mini-arcs or extra battles during the Culling Game where Megumi tames Piercing Ox, Tiger Funeral, or Round Deer.


While we're on shikigami, expand on his ability to fuse them together so that we could get more creative combinations. Like imagine him combining Nue and Great Serpent into a massive, flying electric leviathan, or merging Max Elephant with Piercing Ox to create a crushing stampede of high-pressure water.


Finally, he should've completed his DE before the Shinjuku Showdown so that he could trap enemies in his ocean of shadow, which would include completely cutting off their footing. Imagine Megumi seamlessly morph his own body into his own domain, creating perfect shadow clones of himself, and manifesting any combination of Shikigami instantly from every single angle.


The narrative bottleneck of Megumi’s character was "With this Sacred Treasure" as the ultimate IHYLD move. Instead of Mahoraga, Megumi needed to sacrifice narratively potent Shikigami so that the power could merge into his remaining beasts *(or preferably himself)* to construct a Mahoraga-like entity under his control. Or, if that isn't good, show Megumi duking it out against Mahoraga alongside Yuji in a desperate bid to actually tame it.


\##Shinjuku Showdown


*Megumi needed to be more of a saboteur fighting Sukuna from the inside out, culminating in an internal rebellion that directly enabled Sukuna's defeat.*


When Yuji entered Sukuna’s soul footprint to reach Megumi, he shouldn't be so immobile and useless during the final battle, even if it was "realistic". He needed to be actively fighting whenver Sukuna goes all out so that his output fluctuates. This could even go all the way back to the Gojo v. Sukuna fight, with Megumi using his own DE from within the shadow of Sukuna’s soul and destabilizing the barrier from the inside, giving Gojo a tactical opening.


Even though Sukuna tamed and destroyed many of the Ten Shadows, the core rule of the technique is Inheritance. As Sukuna’s physical body weakened from Yuji's raw hands, Megumi could sever Sukuna's access to the remaining Ten Shadows, channeling the power of the destroyed Shikigami directly into his own soul to protect his mind from Unlimited Void.


The climax should've been a coordinated strike between Yuji and Megumi against Sukuna. In the final moments of the fight, as Yuji prepares his ultimate Black Flash, Sukuna's own shadow could rise up and chain his feet to the ground, preventing him from dodging. This could give Megumi the ultimate hand in his own rescue and Sukuna's defeat.

Overall, what would you have done to make Megumi Fushiguro a better character