I was originally thinking of a Sengoku Jidai unit for the Otomo, and it was a tossup (in my head) between cannons that shot fire projectiles (a mix of fire projecting mangonels and cannons) and basically, the Black Ship.
If you do give them the Black Ship, I wouldn't make it more powerful than the existing one. In fact, i'd make it the same or weaker, cut the upkeep, and limit them to one of the darn things. My Otomo-specific Nanban Trade Ship is just a clone of the Mori Nanban Trade Ship. I cheated a whole heck of a lot in my sourcing, cloning vanilla units of other clans, which is why i'm not fussed if someone borrows what i've done
You may just give them a jumped up Trade Ship instead. My fleets of Nanban Trade Ships just wrecked whatever came against them anyway..
Wako: trade ships with no escort!
Otomo: trollface.jpg
Wako: ffffffffffffffffffuuuuuuuuuuuuuu
Was there supposed to be a cap on the # of Nanban Trade Ships you could build? Because I think I totally forgot to put one in on the Otomo version...