As several comments have already leaned this direction, it seems like a "world history" theme might go over well. Folks have been interested in historical/famous structures in terms of the Architecture line and the whatever-they-call-it line with the Taj Mahal, Tower Bridge, Eiffel Tower, etc., not to mention all those interested in a WWII playset or something. With the announcement of the upcoming Adventurers-Egypt line, at least a couple folks suggested it would be even cooler if it were more realistic, pitting ancient Egyptians against the Romans, or Rome vs. Greece, etc.
So maybe a "World History" theme could offer battlepacks (Romans/Greeks, Allies/Axis, Union/Confederacy, etc.), military vehicles and playsets, vikings, explorer ships and playsets (Magellan, Columbus, Lewis & Clark), samurais, ninjas, the possibilities are endless. Surely LEGO could come up with a certain direction or angle that would make it profitable to both kids and AFOLs.