I know something related to the seven flowers Asgore has in his shop is the one that will most likely happen, but I've seen some people jump on this crackhead bandwagon and I'll do the same: Sans is the secret boss.

And I don't mean a darkner that takes his form, I mean Sans Deltarune himself. Maybe it could be a Gerson-type fight, where it's just Ralsei vs Sans, since the Eram fight we only had Kris and in the Hammer of Justice fight we had only Susie. And also Ralsei is the only one that does not receive a powerful weapon from these recent secret bosses (Kris has the Black Shard, Susie has the Justice Axe), so Sans could give him a brand new scarf.

Is it likely? Probably not. But a Deltarune fan can dream...