I have been playing all of your games up to Spartan, and I found that the most enjoyable of them all is Legion, despite its simplicity. So, if you want to create the best Empire-building game, I think you should add more features to give the players the feel of being an Emperor (or king, or anything else), instead of simple tactics and strategy features..
Correct me if I'm wrong, but you don't aim to build wargames, you aim to build Empire-building games, right? Well, you have to look at KOEI's Romance of the Three Kingdoms series, then. Despite its being too number-heavy, it has so many features (too many to be listed here) of an excellent empire-building game. One of the best example of european game which already come close to the series is Knights of Honor.
I don't say that you should "copy" those games, but rather, have a look at their features, and implement your own features which could be integrated to any of your new game(s), for the goal of creating better games which have the "Empire-building" feel to them..
I could give some detailed examples, but then again, I'm not paid for that. I want to be able to play the games, not creating them..
Hope this is useful in any way..
Looking forward to your new games..!