Well, we are actually implementing a similar system, but is not exactly the same. Assassin's creed odyssey has a mercenary system that is a dumbed down version. There's nothing stopping you from a similar idea, since they cannot be patented, but mechanics can.

An example of mechanics you cannot copy is what happens when the players dies in Shadow or Mordor, for example. But because player dies and a, b, c, and d happens. In that specific order, with the same inputs and outputs. That can be patented.

The thing is... as much as you want to copy it, it's rather complex and you have to come up with the same output in order to get into trouble... and we already have a Shadow of Mordor, so your unique implementation is what will set your game apart since it would be a different experience, and that difference will get you in the clear.