I agree with the OP. Gangplank's R feels like a global, weaker version of MF's E.
It would be nice if it dealt damage every half second and the ult animation returned similar to what it was, with a stream of cannonballs which didn't hit all at the same time.
I'd like also a slow% increase with rank (25/30/35 if not higher) and a damage modifier based on the distance of the ult from GP (like, if he is toplane, an ult near him should deal 125 % damage, an ult mid should deal 100 % damage and an ult botlane shoul deal 50 % damage).
He could pay with a slight CD increase. This would enhance both his laning phase (higher damage in 1v1s top) and his teamfight potential (higher damage in TFs he's present in, higher utility with stronger slow) while toning down some things of his kit which might feel frustrating, like botlane pressure from his presence in the enemy team and light objective control even if he's ten miles away.