So, now that the Friend Captain system has been updated, here's a couple pointers:
-Try to keep a different captain in each of your eight slots. This allows you to offer the most help to your friend list. Any strong lead will do, but if you want some pointers for what to keep in mind:
1. Legends!
2. Traditional everyday captains like Mihawk, G3, Dofy, etc
3. Whatever's useful for the current fortnights. Right now that would be Free Spirits (Sabo, Int Iva, Calgara) and QCK units (Raid Enel, Ace, Jozu).
4. Turtle Leads! This is more important for Mon/Fri, of course.
5. Raid captains! When Duval comes around on Friday, be sure to have relevant captains like Blackbeard, G3, Mihawk, Whitebeard, etc. available.
-I know there's been some concern that the new system doesn't help you with differing captain pairs, like Mihawk/Dofy, Boa/Marco, Kid/Aokiji, or any other set you might want. There is a little trick you can do to get around this, though it does require you to personally own both captains you're trying to use (too bad for Boa/Marco fans). Let's say I want to pair my Legend Marco with a Raid Enel, but I don't see any Raid Enels currently on my list.
1. Set both Marco and Enel as captains on your list, and set Enel as your active captain (this is whoever shows up on the bottom of the screen on the island selection).
2. Go select whatever quest you're trying to do.
3. Since Enel is your active captain, you'll pull up any Enels that your friends have elsewhere on their captain lists. Select one!
4. Once you're on the final confirm step for the quest, you can switch over to your Marco team before starting. Now you have a Marco lead with an Enel friend!
Not sure how often that'll come in handy, but it's a thing you can do.