Trade should only be possible with empires that share a border, or are withing scanning AND hyperspace range.
Building further on my 5 lvl diplomacy post, how about allowing trade as a bonus on economy.
Trading would be only be possible from lvl 0 (neutral, default value), encouraged in lvl 1 (friendly, multiplied by 1.5) and backed by the empire in lvl 2 (full allies, multiplied by 2).
Trade would be impossible in lvl -1 (hostile) or lvl -2 (at war).
Trading between 2 empires obviously has a bonus for both, but the bonus would be relatively bigger for the smaller empire. That alone would balance the game.
Multiply economy and industry for each empire, take the average for both empires and use a multiplier between 0 and 1. Add this number to the income for both empires at the end of every cycle.
This would mean that an alliance of smaller empires would be able to withstand a bigger hostile empire. Another balancing consequence.
What about trade routes. I think ST:BOTF had them. They cost money to set up, but everyday you got a cash bonus based on the resource value of the two allied worlds
Naah, I think that would be to specific.
Perhaps trade wouldn’t immediately jump to a certain level, but would gradually grow until it reaches a max number. But I prefer a bonus on the economy.
However, as trade also brings in knowledge about new techniques, it could also have a (smaller) bonus on industry and science even.