I'll give it a go. So, by "next pirate" I assume you're trying to gather ideas for a team involving Gangplank and MF, correct? Given that MF is a ranged AD carry, and GP's a melee carry/bruiser, this team would need an AP carry, a support, and a jungler/tank. I'm gonna go for the last one.
Bilge- the Brawler at Sea
Passive- Fighting Spirit: Bilge returns 15% of damage taken as physical damage, before resistances are applied.
Q- Brutality: Bilge makes several thrusts with his harpoon gun in a cone in front of him, dealing more damage to enemies the less health they have. Bilge restores health for each kill made with this ability.
W- Harpoon Shot: Bilge fires his harpoon gun in a line. If it hits an enemy, it latches on, slowing both Bilge and the target and changing Bilge's skills. Q will pull the enemy back to Bilge. W will pull Bilge to the enemy. E will rip the harpoon out, shredding armor. The line lasts for 15 seconds, or until terminated by the usage of one of the abilities or the enemy moving out of range.
E- Swagger: Taunts the target, forcing them to attack Bilge for a few seconds.
R- Fighting Stance: Restores Bilge to 100% health, applying heavy armor and MR buffs, but also a heavy slow, that diminish slowly over 30 seconds.
I'm thinking he would probably be manaless, and while the CD's on most of his regular abilities would be normal-ish, the special ones he gets by using his W would be longer, so after he uses his pull, the next time he fires his harpoon, he might have to use his lunge or shred.
The passive and Q are jungling abilities, because the first time I thought this up he'd be pretty useless in jungle. I think this compensates. Maybe he's mad OP now, though. I dunno. Ratios could always be adjusted.
I've also got an interesting idea for a support, but I'll save that one for later.