So, I was just thinking.. And I got an idea for how the Corsairs of Umbar may be implemented.. 
 But, before I mention the idea, let's take a look at how the balance of the corsairs is: 
Stats.
Health: 400
Melee damage: Same as Gondor soldiers (only no forged blades)
Ranged damage: Something balanced and not op
Speed: 70
Now, the idea of how they get added 
 
                                                                                                                        What if mordor could build a tavern on a settlement (Like cirith ungol)                                      It would probably cost 400, and it would be  able to buy corsairs of umbar  

But that's not it 
 
                                                                                                                 It would have a unique upgrade.                                                                                              The unique upgrade is this:                                                                                                     Once the tavern reaches level 3, the tavern will gain a  new ability                                        
 Called "Plunder 'em all"                                                                                                            
Once this is active, for 30 seconds, all corsairs will steal resources whenever they hit enemy units/buildings.                                                                                                            Note, this will only affect Corsairs of umbar 
 
                                                                           What do you think? 
 
 