While I would buy any ToI product that FFG decides to put out, my preference would be for a Pacific Theater expansion.
Don't get me wrong, the Eastern Front is awesome and I would love to see more all-tank scenarios such as Operation Crusader from Days of the Fox, but I think a setting that focuses on the War in the Pacific would really help distinguish ToI amongst the glut of WWII wargames out right now. It seems that everybody is doing the Eastern Front, but not as many WWII games have tackled the Japanese (and the Marines). Even AAM, which encompasses a fair amount of variety in its unit list, has short-changed the IJA.
From a sales standpoint, if FFG times it right they might even be able to ride the wave that HBO's $250 million "Pacific" will surely create. Plus, I wonder if it would be better to wait and make the Eastern Front a big boxed set like the main ToI game, almost like a 2nd edition where new players who weren't interested in ToI's Western Europe setting and were discouraged by the rumors of imbalanced scenarios might be enticed to jump on board.
Anyway, just some crazy thoughts.