What if there was a skill that allowed players to summon monsters from underworld?
Maybe the player would have to draw summoning circles on the ground by using special tiles or arranging items in a certain way, with different configurations for each monster.
It could be a new skill with the players having to figure out the configurations for themselves. There could be a success/fail rate based on your skill and maybe you lose health or all the recipe items if you fail. It would be more expensive/tedious to summon the more powerful monsters of course.
You could summon monsters into your bases or mat traps to help defend them, or summon high level monsters during a raid, or just summon them into galebrook to spread havoc.
A possible downside is that maybe it would make completing the skilled hunter quests too easy. I think it could have the potential for a lot of fun though.