I'm not sure if this has been mentioned already or not as I'm only on page 65 and trying to catch up. Back there wave 2 possibilities are being discussed. My idea is what if Dracula/Vampire Lord wins and achieves his goal of eclipsing the sun? It would open up a 2nd wave for new monsters, sets, and monster fighters. I'm a big fan of Atlantis and aquaraiders and all the underwater themes so maybe Dracula starts hiding the gemstones, as they're the key to both starting and stopping the eclipse, so he sends the ghost ship off with some of them. The ship gets one, and the captain has already hidden a few. Back to my love for underwater themes, maybe one of them has been thrown over board thinking the fighters could never possibly get to it, but the inventive fighters come up with a steampunk-ish looking submarine to dive down and get it. Maybe an ancestor of Brains built it. Anyways, the swamp creature could either come back or we could meet some of his underwater monster friends.
Just my thoughts. Also, this theme seems very close to Monster Squad. An amazing movie if you've never seen it. It's many gems instead of one, and they're trying to eclipse the sun instead of opening a monster portal. That could also be a good idea for wave 2. Maybe Dracula is looking for reinforcements.