Spoilers for My Hero Academia's Final Battle arcAs My Hero Academia's biggest villain, Tomura Shigaraki has gone through a multitude of forms, some where he's more himself than others. His most distinctive form, however, isn't one he'll be able to fight with--it's only appeared in fantasies.
Shigaraki's transformation into My Hero Academia's ultimate villain has been a long time in the making, and to emphasize the transformation he's undergoing, he's long had a particular motif associated with him. That motif is of the caterpillar's metamorphosis into a butterfly--admittedly a bit silly for such a deadly villain, but an effective visual metaphor that's been with him all along. As Shigaraki's transformation has progressed in My Hero Academia, the imagery has changed from a caterpillar, to a cocoon on a branch, to Shigaraki breaking out of the cocoon, until finally emerging with massive butterfly wings on his otherwise standard humanoid body.
Shigaraki's Final Form(s)
In the manga's most recent chapters, Shigaraki has been in a battle for control over his body with All for One. He's gone through several different forms as the two fight for power, riding disgusting waves of mutant fingers and covering his full body with bizarre finger armor. Tomura recently reclaimed his body and is in control as of chapter 379, in a form which he burst forth from like a butterfly from a cocoon, still carrying with him the dead skin from previous transformations around his waist. While Shigaraki's forms have varied from nauseating to vicious in My Hero Academia, none of them have felt very "final form," so to speak.
With the prevalence of the butterfly imagery, though, it's hard not to wonder: why not make that literal? Sure, Shigaraki is already capable of flying/hovering, but at least the elegance and imposing size of the butterfly wings would give him a bit more of a presence on the battlefield than he currently has. Due to all the transforming and mutating and fighting for control, there's very little in the way of "costume" left on Shigaraki's body, so one way to augment his design in spite of that fact might be to give him some more inhuman elements. Butterfly wings are even a valid option as a Quirk. Why couldn't All for One just happen to have such a Quirk inside it?
A Secret Meaning to the Butterfly Motif?
As the battle with Shigaraki continues, Deku's concern hasn't been to kill Shigaraki so much as "save" him, even as the villain insists there's nothing to save. The butterfly motif might actually be a hint that he's telling the truth--after all, a key element of a caterpillar's transformation into a butterfly is that it can never go back to being a caterpillar. Shigaraki may have been an innocent child once, but that doesn't mean he can ever go back to that again, especially after all the suffering he's caused. Deku can try all he wants to force that transformation to go in reverse, but some things are just impossible.
While Shigaraki's butterfly-themed form may never show up to battle, it's still been an important part of how the character has been portrayed throughout My Hero Academia, and may yet hold some significance even as he allegedly takes on his final form.