I remember back in the good ol’ days of Mass Effect 3 , there was a multiplayer Co-op mode where you could select which kind of enemies to fight , and I also remember there was a free DLC Pack that allowed you to fight the Collectors.
Theorically , according to the ME lore , Collectors would be all be dead by ME2 , but that didn’t break the immersion at all.
They could do certain Maps set in the Locust-War.
I bet dozens of people would love to see Maps set into the Hollow/Nexus , or into old Jacinto , Anvil Gate etc.
Or they could simply allow the player to choose what enemy to fight , having Locust , Swarm and DBs as different categories.