Here is the issue:
1. The beginning of the game is extremely fun. You're constantly thinking: what's the best method to get the most colonies? That's a challenge.
2. When war begins, that part is fun too. How do I not get owned by the AI on Suicidal?
However, there are two patches where the game gets EXTREMELY boring:
1. The period BETWEEN the colony rush and the beginning of war. Due to the way the pop mechanics work, you simply CANNOT invade your neighbors because your worlds don't have enough population and production. The AI is in a similar bind. In this period all you're doing (literally) is building factories/research buildings and constructors. That's it. You aren't doing anything else. You CAN'T do anything else.
2. The period AFTER you've attained dominance. There isn't a challenge anymore. The rest is just mop up until the game officially says "you've won."
The larger map, the more painful those two boring patches become. So.... thoughts on dealing with this?
I have one major idea:
Change population mechanics for different races. For example, more warlike races like the Drengin are given a better Birthing Subsidies then other races, allowing them to build up their population FASTER than normal in the middle game. Not as fast as the synthetics, but faster than everyone else. In return make their late game nerfed. Consequently, their play-style is to rush their neighbors with transports BEFORE their neighbors are ready. Teach the AI to do the "Drengin Rush," or the "Yor rush" (for synthetics) and the game becomes much more interesting.
For Benevolent races, make them weaker in the mid game but give them much stronger stuff in the late game. Pragmatic is somewhere in between. Not only can this make the mid game more interesting, it will also add depth to the AI.