Teamwork is deffinately something I'd like to see more of this game- an MP, it should be central. The first thing you gotta do is make people want to join together. There are several ways to do that...
First of all, what we want is to make it very profitable, but just as - or almost- as profitable to do alone as it is with an entire group. How can we do that? Well, here's how: In a battleship fight, it should be extremely hard to take that sucker down. It should be ESSENTIAL to take it from a variety of directions at once, and to not be able to just use a few sunslayers and, boom, it's dead. That will encourage people to aggregate into groups. For another thing, the main payoff should come in LOOT- tons and tons of valuable loot coming from that battleship corpse, so that if three people took it down, they would get just as much loot as if they had destroyed it themselves. That will prevent the want of going solo.
Now, I don't know if it's even possible to have more than 1 loot off of one ship; but, if not, a variation on this idea could work.
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"Here, use this poetry."
-Death to the Extremist