A skill for trading would be nice.
About the rest, it's silly. Mercenary joining the group is something I only saw at legendary moonlight sculpture (manga), but that's beside logic. Who would give his life for getting items first*?!
*except Apple Users in front of the applestore waiting for a new iPhone (SCNR)
That's true, but mercenaries already do die for just a few hundred gp
I don't mean they'll join your group the same way a companion does; just that they'll teleport back to the city where you hired them in and stick around after you have them join your group; making them semi-permanent npcs. They would still cost money to hire after they joined you-just not as much as they would normally,
The benefit to this would be that you could safely give them armor and weapons without having to worry about losing them if they die.