Gangplank will get a rengar sized rework, and let me tell you why. NOT THAT I AGREE WITH THIS STANCE, BUT THE FOLLOWING IS RIOT LOGIC.
Passive: The slow from this will probably be removed/Moved. Stacking on-hit slow is toxic because it's useless when behind, and punishing when ahead (enemy has difficulty escaping.)
Q: This will probably be changed to a skill shot, but have almost all aspects of it buffed. (Like rengar's bola change.) Q as it is now can only be overpowered or underpowered because it's high damage targeted harass.
W: This probably won't be changed much since it's GP's signature ability. If anything, they will change to to be %missing health heal because Riot has constant lane sustain because it removes incentive for interaction.
E: This will be completely reworked, because like Warwick's attack speed buff, it gives a lot of "hidden power" while not having any interesting gameplay behind it. Riot will probably put SOMETHING with a speed buff here though.
R: This will either keep it as it is now, OR just make it even less random. Other than that, they will probably leave his ult alone.