Inter-Empire Population Exchange

Right now it seems like the only way to get population units from another empire is to either conquer their territory or use diplomatic pressure with the demand of Force Immigration.

I propose two separate, additional ways that population exchange could occur-

1. Border systems. If two systems from different empires are adjacent/have touching borders, and the empires are not at war/closed borders, there should be some small chance of one system spawning population units from the other. This chance should increase if the empires are at peace, with even higher chances on trade agreement, science agreeement, being in an alliance, and having higher influence output than the other. Perhaps there could even be associated laws/diplomatic agreements related to increasing/descreasing migration.

Overall, 70-90% of the time the spawned population should still be based on the system itself so the player has control over their own growth, but this provides a reasonable chance of catching population units from a neighbor.

2. Direct trading. Two empires agree to exchange some population units. This could have multiple possible implementations, each with their own advantages and disadvantages-

A. Grab the agreed units, and set their destinations, based on some algorithm out of the players' control.

B. Player choice-

B.i. Players use space stations to determine where to grab units from.

B.ii. Players choose where they want new units to go OR the sending player chooses where they go.

B.iii. Players must immediately figure out these choices, and cannot otherwise end their turn OR players have some turn limit by the end of which they must have sent the units they agreed upon, and no units are sent from either side until both have met their obligation.

Additionally there could be diplomatic consequences for going to war/cold war while the populations are still being exchanged, and even events/massive disapproval for destroying the related civilian transport ship (after all, they used to be YOUR citizens).

Edit: A possible third way- Empires with the "Slave Driver" trait can sell non-main population units on the Market, and buy any population units available. This could also open up NPC-generated slave units, explained away as being sold by either the more aggressive Minor Factions or the Pirate League.