Immigration treaty - The target empire's dissent and unhappy pops would move into your empire, with the accompanying benefits. (This option would be popular for players and computer controlled empires looking to get species that can colonize worlds their species cannot, without needing to gene mod or terraform; would also be particularly attractive for Xenophiles of course!).

Emigration treaty - Your dissent pops will move out to the target empire, ridding your empire of traitorous citizens without needing to resort to actions which cause diplomatic penalties. The presence of a large diaspora of your empire's former citizens in another empire's planet could also lower the 'Cede planet' and 'Liberate planet' war goals for that specific planet. (This option would be popular for players and computer controlled empires that seek to get rid of dissident citizens without angering other empires, particularly Xenophile Fallen Empires, by purging; it would also be useful for spreading your species to other empires and thereby lowering the war goal cost for those planets; and also useful for Xenophobes who want other species to move out of their empire).

Inter-migration treaty - two-way treaty that is currently the only option available post-Asimov. (This option would be especially useful for building trust between empires).

One-way migration treaties should be re-enabled as they are realistic. In Earth's recent history, nation-states in the New World have essentially always allowed legal mass immigration from the Old World while the opposite was not true. Even today New World nation-states allow legal mass immigration while Old World nation-states do not, with the exception of Western European countries as of a few decades ago.It's entirely possible that a conformist authoritarian empire might be happy to let its dissident citizens leave and a free democratic empire might be happy to take these oppressed masses yearning for freedom in, without the opposite being true.This already has an inherent balance as the source empire no longer has to deal with dissident pops and can get rid of them without having to resort to actions that will cause diplomatic penalties (i.e purging) and the destination empire will have to deal with pops of wildly differing ethos who may even rebel and cause them trouble, but if necessary it can be further balanced out by a temporary 'Culture Shock' negative happiness modifier (-10 to -15 perhaps) for immigrant pops.The migration treaty mechanism would remain in the diplomatic interactions main screen, but pressing the button would now lead to 3 options: