It's a fiction bro, everything can happen if the author wants it to.
We know nothing about the 10S full potential, its "perfect" DE, the sort of skull (or whatever it is) in it etc...
I don't think this "stuff" inside the chimera garden is here for nothing. Let's fanfic a bit: It could hint the possible existence of some ultimate shikigami, stronger than Mahoraga/Rika. Possibly the merging of all shikigamis into one (chimera: "in Greek mythology, a fire-breathing female monster with a lion's head, a goat's body, and a serpent's tail").
Moreover, Makoda deer can provide external RCT help, so other Shikigami(s) could maybe provide external CE/CE output supplie by exemple.
Besides, the shadows themselves are a total mystery. It allows the user to hide himself and other stuffs, it also allows him to create shadow clones, and there should be more than that.
So let's have Megumi awakening his 10S mastery for some random plot reasons and get a full control over all its skillset/possibilities, and you could have a stronger 10S user than current Sukuna is.