It would be interesting if occasionally we would find an NPC that if we defeat it, it would give an artifact "mercenary contract" that when used on a planet it puts the NPC as a defender of that planet for 1 week.
Also, a simpler version of this is when the mercenary contract is used it grants a "mercenary guard" bonus to the planet of attack, defense, and cloaking for 1 week.
Different versions of this could have variations in attack, defense, and cloak focuses; or even various levels such as Inexperienced, Special Ops, or even Mercenary Fleet.

I love this idea that would be a great drop reward. You could have a uncommon that drops the inexperienced, a rare that drops the special Ops, and an Extremely Rare boss type NPC would drop the Mercenary Fleet. Maybe make the boss for this one a bunch of ships that spawn once the first one died so you can have it like 25 more ships to go and each has X amount of health. Or have them get harder as they come up. I think this would make boss fighting more interesting. You could still do the awards the same based on damage as well.

I like the idea...

I like it , but what kind of stats would it have?