I'll start first, since ako man ang nagsugod I say Zombies ang sunod na Structure deck. I based it on the rumors and the Structure deck releasing cycle.
Why Zombies?
Many Zombie monster cards and support cards must be reprinted, plus majority of the strong Zombie cards are all promo cards (e.g. Mezuki, Burning Skulls, The Yokai Archetype and etc.)
If we analyze the cycle on SD release, before Zombie World, there was SD 14*(forget the name) and before Zombie Madness there was SD 1(dragons). Now if SD 22: Dragonic Legion is a combined version of SD 1 and SD 14, I could predict that Zombies will be next.
Rumors has it that it would center on A Burning Swarm Strategy Zombie deck(like the burning skulls).
Anyway its just only rumors, so take this as a grain of salt.
What's your predictions?