While I'm not sure I buy the theory about Katsuki ending up as a vestige in OFA, I'm still going to throw this theory into the ring since it would offer some weight to Toga's interference.
What if the reason Katsuki is seeing All Might's vestige is due to Izuku rejecting the ides of sharing the same heart and mind as him? This then triggers vestige All Might to see out of the person who Izuku would want to share those things with. While the space we see Vestige All Might and Katsuki in isn't OFA, I do believe the timing of these events are meant to line up to the scene we just say with Toga. What is it that Izuku notices as he's flying back? Could it be Katsuki's spirit hurdeling towards him? We'll just have to wait and see.
Reguardless, the placement of events is odd for Katsuki of all characters to have truly kicked the can. There's so many possibilities in how he could be brought back either through OFA, the second user, Eri, or if Horikoshi really wanted to, made it so Izuku had a quirk all along (though, I'd prefer not to see that) and healed him. The beautiful thing about MHA is the plot loves its misdirections and it's only fitting to see how the rest of this situation will play out.

















