What I want to try for a tweak is this:
If there are Mysterious Islands in play, Mercenary ships can start S-away from one. However, you can not do this if there are any ships in your fleet which do not belong to the Mercenary faction. (Ex-pats on a ship that originally belonged to another nation are fine.)
(Alternatively: If Mercenary ships have their linked crew aboard, they can begin the game S-away from a Mysterious Island)
Mercenaries are entirely battle focused, and thematically they are defending their little utopia from invaders, so this would help with both of those things while tying into their stateless rebels lore. I really like the Mercenaries and just bought a bunch of them so I’ve been thinking about house rules for them a lot lately.