My guess has been Amy Klobuchar (MN Dem Senator). She’s been positioning herself in power positions (albeit just under the radar) and is frequently on the usual media outlets. Commie through and through, but with a lesser amount of baggage than most and the gender advantage.
She is papabile, and she does better if she doesn’t have to compete in primaries.
But then, what do they do with Kamala? Of course, she’s a problem in any event.