So far, there have been mods made or being made for the Middle Ages (SAM), 30 years war (P&S), 7 years war (EW2), Napoleonic wars (HEW), Civil War (HDN), and Franco Prussian war (my mod). Therefore, there are only a few logical options left for new mods, including the ancient times (with Rome, Greeks, Carthage, Persians, etc), the Japanese civil war period, WW1, and WW2. After I finish my current mod, I think I will want to make one of these. Which one would be best in terms of popularity and gameplay?
I like the ancient times idea, but there are heaps of periods you could chose to do a mod on, you should also think about what era interests you the most, as this is what will make you passionate about making a mod for that era.