I've wanted to do that in my own game but I'm not great at modding outside of the general cut and paste and value changing bits. The way I've thought would be nice is if characters had a 'sell' icon next to the 'destroy' icon. That way if a character is in your own land you can sell him for about a quarter of his price and he becomes one of the mercenaries that any player can hire. Now if you hit 'sell' on another persons land then a message pops up offering the unit to the other player for a calculated price. Perhaps mercenaries would have a 1 gold upkeep too just to keep people from abusing them, although at the same time you would have to make sure that the computer wouldn’t burden itself with an army it can not support just because you keep offering him units every turn.
Lastly regardless if people sell their own units or not I would love to see unique ethnic units pop up in the mercenary screen such as ronin, head hunters, Venetian crossbowmen, highlanders... basically all of them unique units of the nations that are not in the game.
Hopefully the 'fractured star' mercenary screen will provide a jumping off point for a skilled modder, although I'm still starting out so I know that wont be me unless people are willing to wait until I do a couple of less ambitious mods first.
