Back when Noelle was first introduced, I speculated about Noelle transforming into an Endbringer, but quickly dismissed that because Leviathan was never human according to Tattletale’s power.
But – and this is a long shot, more an idea than a solid theory – what if she’s not becoming an Endbringer so much as incubating one? That way, she would be the source of a new Endbringer, but that Endbringer would not have been human.
Or maybe she’s becoming or incubating something entirely different, something even the Endbringers see as a threat? I don’t think it’s the final threat Dinah predicted, but it might be a big one.