Here's an idea.
Let's say you have five workers and you want to build two things. Perhaps we should be able to assign to workers to one building construction and three to another. Maybe each extra worker assigned could reduce the construction time by 10% or something.
I believe this would be a good idea for the game as it would make having multiple workers more of an advantage even for singular objects, rather than having to need five individual tasks to get the most of your five workers.
As you have already paid gems to have more workers, I don't think you should have to pay to unlock this feature. It can be unlocked at a specific level or for everybody in the game.
Thank you for your time reading, Apep.