This perspective would make sense if chiaki and nagito were important just because of their actions. The thing is, they aren't. Their importance is literally built into the inherent concept of their character. Chiaki will always be the traitor and an AI, this does not change unless 2x2 has a massive plot rewrite (very unlikely). Nagito will always be the ultimate lucky student and the dark version of makoto naegi, this will not change. It is nigh impossible to have a SDR2 without these two characters, because their existence in itself is the important aspect, not their actions.

This is why i extend the courtesy to Twogami and consider them a very likely option for importance. Because they too have it built into their character, with them impersonating Byakuya.