Marcone has yet to demonstrate the one quality it is explicitly stated that Starborn have: the ability to wield power over Outsiders. For that matter, we haven't seen Elaine do it either—unless we start really stretching things, assume Aurora was infected as stated by Lily, and that the brambles and thorns Elaine held back was some kind of Outsider-powered thing (in fairness, it does kind of look like Aurora made something from nothing, but hey, it's the Nevernever; the rules for magic don't necessarily apply).
Aside from that, I third (or fourth or whatever) Mr. Death's statement that Marcone becoming super makes him less cool. Aside aside from that, Marcone would only take power if it came without strings; Marcone works for Marcone. If he picked up a mantle, for example, it'd come with obligation, and that's just not his style.