[Suicide Junkie]

That would require a 900% maximum trade level, and would take 900 turns of solid alliance to achieve.

It would be quite possible for two players who ally right from the start to specialize...
One player could colonize everything and have a race with massive resource, research and SY bonuses, while the other takes massive combat and maintenance bonuses.

Player 2 would no economy except the trade profits, but be incredibly deadly in combat by using player 1's ships.
Player 1 would have a powerful economy, protected by player 2's combat skills.

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I suppose you could also make a mod where critical technologies are restricted by racial tech.
Each race would only be able to choose one, and would have to trade with the others to get their basics.

EG: Resource extraction facilities, ship construction, and research/intel.
Race A would trade resources to Race B for ships.
Race B would trade ships to race A so they could build and maintain a fleet.
Both would trade with race C for research and intel points.

In that way, each race would be nearly helpless on its own, and having those "alliances" set up would be 90% of your strength.