After reading quite a few threads, and playing pretty much exclusively huge/gigantic galaxies, i would like to suggest that a look at how the ai desides what is a useful ratio of transport to regluar ships might be in order, as a way to increase difficulty, especially in regards to late-game (when most techs are researched by all civs that have survived the Mid-game wars)

As an example, in the current game im playing, the most powerful enemy (status with me, at war) currently has 100+ planets, 923 total ships.. but only 4 of these are transport vessels.(and if i counted right, only four or five planets acctually building transports) In a tiny/small/medium galaxy with a limeted supply of planets, this would be fine - 4ish transports should be able to take out a planet once defenders are gone, and obviously in those galaxies, one planet means ALOT more relitively speaking than in a gigantic galaxy.

This ratio is representative for just about every game ive gone through, the only exception being the Korath ai, who do to thier spore techs, seem to focus more on building invasion ships. Increasing the ratio of transport ships to normal attack ships would greatly increase the AI's ability to both increase its teritory and defend its teritory (by taking planets back) - meanwhile making the human player value the use of "defensive" fleets alot more.

Id also like to suggest that the dev's take a look at the way range mechanics work on the larger galaxies, restricting the "base" range. This would in essance force the AI to make better decisions simply by restricting thier over-all options.

end 2bc,
thanks for the great game by the way.