Or maybe as a alternative there is a goblin shop where you can buy new tiers of goblins at the cost of selling slaves.
At the Goblin Shop
1 slave = 1 melee goblin (use in case you run out or need more)
2 slaves = 1 archer goblin (use in case you run out or need more)
3 slaves = 1 shortstack goblin
5 slaves = 1 Hobgoblin/tank goblin
5 slaves = 1 wolf-rider goblin
10 slaves = 1 spell caster goblin/human
To add to that, if the dismantlement feature from MBM is added to the game, we could harvest valuable parts of slaves to sell/trade with, or just sell the slaves themselves, to evil individuals/groups for money, slaves, or useful items/equipments.
Possible items/equipment that could be obtained:
1. Defenses attachments like unit boosting totems or enemy debuffing items
2. Base repair/fortification materials
3. Single use items to use on monsters, slaves, or enemies
4. Items that helps in breeding and growing
5. Aboveground base building materials
6. Unit Stats enchantments
And the way these evil people come to you depends on your progression in the game.
In the early stages, goblin tribes will visit you to trade your slaves for some of their unique goblins, like you said.
After surviving a few human raids, cultists will take notice of your infamy and ask you to trade some of your slaves and monsters for handy items/equipments or unique slaves.
When your reputation increases, black market merchants will take an interest, and you can sell your slave/monster parts for money and buy stuff from them. They appear and stay near your base for a while before departing.
And after reaching a high level of infamy, corrupt nobles start appearing, requesting specific slaves/monsters in exchange for money or high-end items. The request doesn't have to be done immediately, they will wait for a few days before returning to collect the slave/monster.
Refusing to trade multiple times can cause some effects to happen:
Refusing goblins enough times and they will increase the number of goblins in the next raid. Only that wave will be increased and not later waves.
Refusing the cultist enough times and they will give you a random debuff lasting 1 day.
Not trading with the merchant would lessen the time they stay at your base or downgrade their stock.
Failure or refusal of the request can cause the corrupt nobles to send a wave of tough enemies to your base once.