GalCiv3 has a extreme performance issues with long games on big maps. Play a game on an extremely large map with many opponents, and eventually, the game will fill your RAM and make your CPU's busy to the point that you cannot play the game safely any more. It will even struggle to move your space ships without interrupting the music. Time between turns increases, the game will stutter all the time, and eventually, it will crash.
This needs to be fixed.
Apart from that, it is a great game as-is, though some espionage mechanics would be nice and one thing I would love to have is a tracking of a planet's population per species: If Terrans successfully invade an Iconian planet, the population afterwards will be a mixture of Terrans and Iconians, and the game should track that. Likewise, tourism, trade and the like will add some minorities to all kinds of planets, with the minorty only partially helping to defend the planet against their "own" government, etc.
