The Last Herald in the puzzle
Now that we have had Air-Walker and Cosmic Ghost Rider release, I might have an idea as to who we might actually get. Assuming that the empty boxes linked to Galactus are Air-Walker and CGR respectively, we are left with the box connected to both Silver Surfer and Galactus. Combined with the knowledge the Heralds of Galactus synergy among the four we already have is shown to have no one else planned to be connected, this leads me to believe that the last herald before the puzzle is updated at the halfway point next year (or another time next year) will be The Fallen One.
The Fallen One is the first herald of Galactus, created out of dark energy. As some of us know, he is a herald no more so that explains why the synergy is complete among Silver Surfer, Terrax, Air-Walker and CGR. The Fallen One is connected to Silver Surfer through that he effectively became The Fallen One when he was overpowered by Thanos and thought dead.
This is just some theory, but who knows if it would actually be true (I hope so!)