With the addition of alliances, I'm under the impression that you can still betray that alliance and kill other players - if friendly fire is disabled between alliance members, this suggestion doesn't really work.
It might be that people take advantage of the alliance system to get a little bit of xp, keep an eye on another ship, and then just kill them before turning things in at an outpost to get all their gold/xp. While I don't think this behaviour should be prevented in game, I do think a "reputation" system could be added which lets people know whether members of a different crew are routinely betraying alliances.
Basically if you join an alliance with another crew, there'd be an option to go to a tavern and ask the bartender about them. "I just joined an Alliance, have you heard anything about these pirates?"
Based on the combined previous actions of the crew, you'd get different responses:
"I haven't really heard much about them, actually."
"Most of them seem like a decent lot, but there's one that's been up to no good."
"Woah, you want to stay away from those cut throats!"
etc.
I don't think it'd name names or anything, but it'd be a fun way to give a rough sense if you can trust your alliance, and whether you should be watching your back.
