On the other side, what do I suppose to do with Slaves which i captured after I switched from Forced Labor? Nobody is willing to buy them, I cannot settle them, they don't improve land, I can only order them to suicide so I don't waste money on their maintenance. Is that what you want us to do?
Why not to offer them refugee? This could result in +1 pop gain and temp. +1 unhappiness from immigration and/or temp. +1 unhappiness from discrimination.
This leads me to the idea of refugee unit. If your war weariness gets too high, you may lose a pop. point in one unhappy city and a refugee unit would appear in neighboring civ's borders. The neighboring civ should be at peace and have emancipation civic. Refugee unit could make you need to decide:
1) to keep the refugee unit and pay high maintance.
2) to build a Refugee Camp improvement that connects the possible resource, but yields in +1 unhealthiness. Refugee camp could be disbanded after being worked by city for certain amount of turns.
3) to settle the refugee and gain a pop point,+ 1 unhappiness from immigration + some neighboring culture. Nationalist civic could double the unhappiness gained.
4) to delete refugee (slave) unit giving you a genocide penalty from other leaders not following a form of totalitarian regime. The genocide penalty could be also triggered when you kill your enemies refugee unit or pillage any Refugee Camp.
5) send the refugee unit back, resulting in some stability loss since you don't live up to your emancipation values. When you resettle your own (domestic) refugee you shouldn't receive any additional penalties.
Refugee units could be also triggered from religious persecution giving the refugee unit to nearest civ with free religion civic or to a civ with the appropriate state religion.
Refugee units could be created in collapse. Some refugee units could appear to a neigbouring civ, but if it is your own civ collapsing, some refugees could appear in your capital as domestic refugees giving you no unhappiness penalties
Refugee units could be also triggered by combination of unhappiness and bad economy. Also, vassal states should have higher chance to create a refugee unit to their master.
This leads to America, where it's UP would be receiving Refugee units occasionally, like it gets pop points today. Also, instead of +1

, settling refugee in America could result in extra commerce or all refugees could be treated as domestic, yielding just +1 pop point.