really big games

If you want a really challenging game, try a rich quadrant. Most of the AI problems stem from a lack of resources. There are several things to be done.
1) In data directory modify SystemNames.txt to have at least 255 system names. Be sure to start each name with a capital letter.
2) In data directory modify QuadrantTypes.txt under name Mid-Life to be chance 1 for all types except standard 3 52 (1 star) Binary 3 21 (2 star) and Trinary 1 13 (3 star). This adds up to 100 86+14x1. You could remove all the variety but it makes the map more interesting if there are systems which need improvement.
3) In data directory modify Settings.txt increase AI values by a factor of ten :
a) Maximum Number of Systems to 255. More will crash.
b) c) d) Minimum Empire Minerals (Organics Radioactives) Generation to 2000.
e) Minimum Empire Point storage to 500000.
f) AI Mega Evil Empire Threshold Score Thousands to 5000.
g) Default Number Of Units Per Player to 10000.
h) Default Number of Ships Per Player to 2000.
4) Create an Ancient Race which sees all the quadrant and include it as the first race in each iteration of the procedure below. Select Game Settings Players Can Save The Map During The Game on every iteration. Generate games until you get a midlife quadrant with about 255 systems and a few waste (black hole nebulae asteroid) systems. It should have about 2000 planets. This is your start point - save the map.
5) Make up a list of races which uses all possible planet combinations (rock,gas,ice) times (None, Oxygen, Carbon Dioxide, Methane, Hydrogen) except Gas-None makes 14.
6) On each iteration add all 14 races and select Player Settings - Home Planet Value as Good and Number of Starting Planets as 10. Load the Lastest Version of the map. Select Generate Random Computer Controlled Empires and deselect Neutral Empires. Select Number of Computer Players High. This adds six of the unselected races to the creation making 20 times ten good planet hits per iteration. Generate a new game, save the map and exit to New Game and repeat. My experience was planets 2158-2325-2456-2560-2647-2728-2787-2843-2896-2954-3012-3032-3059-3113-3175-3212-3247-3293-3323-3327-3351-3383-3412-3443-3468-3506. At that point I had half of the planets I wanted and was tired of the slow progress.
7) On each iteration select the Ancient race and one of the other 13 repeated 19 times. This generates the same planet type 10 times for each of 19 start locations. Save the map and move on to the next race. After the first pass I had 4734 planets. After the 2nd pass 5505. After the 3rd pass 6085.
8) My target was about 7000 planets but I also wanted them evenly distributed amoung the 14 types. So on the fourth pass I modified the race characteristics of the first of each set of 19 to be an ancient race and repeated the race until it had over 450 planets to be settled of the proper type and atmosphere. The end result ranged from a low of 465 to a high of 619 for rock none due to moons. The total was 6839 planets of which at least 4681 were large with about 120 minerals organics radioactives and a good climate.
9) The game changes planet numbering from roman numerals to 31 32 33 etc when it gets too many planets in a system but seems to have no upper limit. I checked the map editor and it barfs when one of my planets is selected, but who needs the editor anyway ? So if you want to revise the map warp points or something with the map editor, do it BEFORE you follow the procedure.
10) You thought the AI was too easy ? Well just try invading a sytem with 10 proper atmosphere planets to blockade/invade of 35 total in the same system. Instead of one dreadnought per system you need a fleet ! You had better expand VERY quickly because the AI will be all over the map with its resource problems solved. My answer is to research more than the AI.
11) You can make the AI emphasize research by changing directory Ai file Default AI Settings item Maximum Research Point Generation from 100000 to 500000 or more.
12) While in that file, you also need to change item Get Angry Over Allied Colonizable Planets from True to False. Otherwise you can not settle more than about 39 planets before they declare war on you, which is about one half of one percent of the planets available rather than five percent in a standard 100 system quadrant. You might also change by a factor of ten the AI anger over planets to keep it in scale by modifying Percentage of Planets to consider as Attack Locations For Anger from 10 to 1 and from 5 to 1.
13) The procedure took about seven hours at 2-3 minutes per iteration. You can test the results by trying it yourself. After a dozen big games with a blitz strategy that Lasted from turn 120 to 150 I got tired of the AI not being able to put up a good fight....