I say Rome. It's way too early for a Victoria 2 expansion to be considered (as has been said before, give it time to reveal the areas that need expanding... and for the kinks to be ironed out), and the same can be said for EU3 and HOI3 (two expansions one one the heels of the other is just weird).
So that leaves Rome. And I've always wanted to try it, but I didn't have the computer for it when it came out and then everyone said it still needed work... Give me a reason to buy the game, please.
Well to be fair, Rome Gold is pretty much picture perfect as far as stability goes.
It's just, the game plays more like an EU-Lite than a complex grand ancient strategy game where you're watching the rise and fall of empires.
It's watered-down, simplified, but fun.
I'd like to see it rival Vicky or EU3 in terms of complexity.
At the game's timeframe things such as religion were nowhere near as crucial as your culture, your policies as a nation, etc.
Hell, I'd pay at least $10 just to be able to have 2 consuls, an accurate Roman Republic (both Pre-Marian and Post-Marian), and the Spartan dual monarchy (to say the least of making the map bigger so we can get more city-state goodness).
If they push the timeline back to pre-Alexander, or even right before the death of Phillip, I could imagine being a city-state like Thebes and having to form a League of like-minded and similar cultures to band together to try and repel Macedonian aggression. Things like that.
Or to be able to have complex political and/or military interaction, such as Pyrrhus briefly taking over command of the forces of Magna Graecia, before moving on to Sicily, and then heading back to Epirus.
There's so much potential for cool stuff. All one has to do is read a little Xenophon, Herodotus, or Polybius to get some inspiration.