You should be able to create a new game with a different race if you unlocked it in the main game, I say this so you can experience which race is the most op or just because you like it better.
Also, I wonder when they will add a goblin like reborn, it would be crazy to be the goblin spawner and lead them to conquest xDDD
But I understand a problem because the mermaid and abomination can mix with humans, whereas a goblin reborn wouldn't have this advantage unless they justify it in some way.
In such a case, the game would have to have a different start with different missions, as well as a different ending, which poses a lot of work for the developer.