I'm surprised they never found the Soryu or Hiryu. I mean, the Soryu was lost right around the same area as the Akagi and Kaga. While the Hiryu did move away from the other three after they were hit.
Here's an obvious one that is criminal we haven't done yet. Armored cruiser challenge. Draw an armored warship of a cruiser type laid down between 1890-1910. That seems like a good idea, or how about a protected cruiser challenge? Something along the lines of either the Atlanta Baltimore or Chicago?