I'd say WWII Ardennes, and I say this from the perspective of someone who selfishly wants this system to thrive and grow. I think Ardennes gives it the best chance to do so. Why?
1. WWII seems to be the most popular wargmaing era. Vietnam and Cold War are more niche markets. And I'm guessing that Bugle games sell better than Pacific ones.
2. The US is the biggest wargame market, so it makes sense to do something with US troops this time.
3. I'm assuming most people who own HoS will buy an expansion, but are more likely to do so if it adds something they don't already have. They already have the East Front (I know, just one part, but still). Similarly, anyone who passed on HoS because of the subject might be enticed by a Bulge game.
4. This will give you a chance to showcase those beautiful Noville maps, which might also attract the eyes of potential buyers. For those who haven't seen the Noville maps, I'm posting a pic below.
5. Ardennes is a perfect campaign setting.
6. Ardennes gives you a chance to introduce a wide variety of units (from SS to Volksgrenadiers, paratroopers to line infantry, and even commandos) and equipment (Nebelwerfers, Me-262s).
But hell, I'll buy whatever you next produce.
