@z911122:
1. Counterpart to Blackbeard.
It's rather clear that the Blackbeard pirates are bigger, stronger, darker versions of the straw hats. The current lineup is something like this: Luffy to Blackbeard, Zoro to Shiryu, Usopp to Van Auger, Chopper to Doc Q, Robin to Devon, Sanji to Lafitte, Franky to Burgess, and Nami and Brook will somehow be paired up with Vasco and Pizarro.
The last member is San Juan; does Jinbei really offer a counterpart to that?
Yes (though i disagree with some of your matchups with the BBs). Since San Juan Wolf is so huge that he's forced to be in the water alot (i know its just from that one pre-TS bb scene, but i assume its pretty common for him to stay in the water), it would make perfect sense for a water-specialist to fight him, particularly since it's one of the best ways to even out Wolf's incredible size advantage. Jinbe's already said Fishman are much stronger in the water, and since on land he's already roughly on par with the Monster trio, fighting the largest living being seems like a good match for him, particularly considering how well he's handled Wadatsumi.
@z911122:
2. Family member
The straw hats could also be thought of as a family. Currently they have a mother (Robin), father (Franky), grandpa (Brook), a daughter (Nami), a pet (Chopper), and 4 sons. Kind of a large family already - although I suppose you could always add a grandma, uncle, or cousin in there…
This is kind of arbitrary, but I assume Jinbe would be an uncle. He sort of seems like one to me, in that he's the Veteran whose been through alot but not with the rest of the "Family", so he fits the trope of the "traveling Uncle".
@z911122:
3. Ship role
This is kind of a smaller complaint, since Oda could always add in a role that isn't normally on a ship (as you'd rarely find archaeologists on a ship). What role would a fishman fill? A helmsman?
I assume he'd be the Helmsman, just from the fact that its an obvious ship role that the SH currently lack and that he's shown he can do. He doesn't have to be the world's greatest helmsman or anything, but if Jinbe joins I think he'll sort of naturally fall into the role of the guy steering the ship. He could also be the "Under-water" or "Anti-ship" specialist as well, since Zoro's role is basically "Guy who cuts stuff when necessary"
@z911122:
4. Attack method
I personally like to think of the group in MMORPG terms. Luffy = single target DPS, Zoro = melee AOE and off-tank, Sanji = Rogue, Nami = long-range AOE, Usopp = hunter, Chopper = cleric, Robin = utility, Franky = main tank, and Brook = buffer (feel free to laugh at the comparison). Anyways, Jinbei and the other fishmen seem to be attacking using the brute force method, which I think Luffy fulfills already. I understand Brook and Zoro are both swordsmen, but I feel like Brook is starting to move away from swords only and is using music a lot now, which could possibly boost the other straw hats when fighting near him.
First XD. I'd love to seem a full 9/10 vs 9/10 Team fight at some point in the series
Second, I'd say Jinbe fits the Protector role. Basically the guy whose job it is to pull aggro off any of your squishes that draws it so that your main Tank can concentrate on holding the rest of the foes down, and basically function as the reserve tank/off-tank that can switch between the roles as the fight demands it. Also, Fishman Karate seems effective for CC (though Robin and Brook seem to be able to pull off that role well as well) since its like AOE ranged physical damage, which furthers Jinbe's qualifications for Protector.