I got another related idea. There should be an onslaught mode for the AI.
That mode only makes sense with the appropiate map and should be available as slow, normal and fast. The AI uses all recources(cheated) to spam constantly growing fleets to throw at you. Also it will slowly rise the tech level.
The initial size and the growing rate depends on the setting metioned above.
This AI should never flee and the spamming world should be insanely fortified(e.g. four starbases and 4 times more tactical struktures. Immune to any Planetary cannon, not defeatable throuhg culture)

Not only SP. How many "who can survive the longest" or Koop-Maps are there for Warcraft 3. To be honest that was the reason i bought WC3 after i played it a few times on LANs.

@ Grotten:

I like the idea of the AI's targeting of Gateway-like planets.

I also like the idea of onslaught in sins.  Perhaps have a map where only you can colonize, but the enemy spams.  Fog of War would have to be off, but the computer would simply send say, 5scouts>5LF's>5LF's>5LRF's>5LRF's>5Flak>5Flak>1Carrier>1Carrier>2 SuCrui>2SuCrui>1HC>1HC>5LF's>5LF's>5LRF's>5LRF's>5Flak>5Flak>1Carrier>1Carrier>2SuCrui>2SuCrui>1HC>1HC

>5LF's>5LF's>5LRF's>5LRF's>5Flak>5Flak>1Carrier>1Carrier>2SuCrui>2SuCrui>1HC>1HC>5LF's>5LF's>5LRF's>

5LRF's>5Flak>5Flak>1Carrier>1Carrier>2SuCrui>2SuCrui>1HC>1HC>5LF's>5LF's>5LRF's>5LRF's>5Flak>5Flak>

1Carrier>1Carrier>2SuCrui>2SuCrui>1HC>1HC>5LF's>5LF's>5LRF's>5LRF's>5Flak>5Flak>1Carrier>1Carrier>

2SuCrui>2SuCrui>1HC>1HC>5LF's>5LF's>5LRF's>5LRF's>5Flak>5Flak>1Carrier>1Carrier>2SuCrui>2SuCrui>repeat 5 times.

 Then add a battleship, repeat another time, add a carrier, repeat, add a dreadnought, repeat, add a support cap, repeat, add a support cap, then just keep running through the loop above.

So...

1. 5s

2. 5s, 5LF

3. 5s, 10LF

4. 5s, 10LF, 5LRF

5. 5s, 10LF, 10LRF

you get the idea...

And, since this is the AI we are talking about, we are going to end up seeing fleets of 16 level 10 caps because they won't get focus fired and will get huge amounts of EXP.  Since only you can colonize, the larger the map the better.  Just make sure to expand quickly because by the time the AI starts adding in caps, it would get pretty hard to expand any farther.  You will want to spam frigate factories on the frontline worlds.  Sure, they will outnumber you, but if you can just replace anything destroyed, it will help you survive longer.  And, of course a few SB's wouldn't hurt...  Your rear worlds would need to be full eco and your forward worlds would need to be entreched to the teeth.  (If Vasari, put some phase gates behind the forward line so your fleet can move.

The game's idea would be to see how long you could survive, but if you had it so the planet was called "stellar shipyard" so the entire thing was just a giant factory (would make sense for this game...).  If bound and determined (on a map with 200+ worlds) you might be able to kill it.  Victory over the AI on this would be very good evidence of being a good player.  Smaller map=better player.