Ok, seriously what is with the nerf bat lately. It's not ok to take 60% base damage off an ability. 15%-25%, ok it's more reasonable. But going from 200 bonus damage to 80 is ridiculous! That is literally the current rank 1 bonus damage on the ability! You have to put 5 ranks (which you'll likely max second @lvl 13) just to get it to the same effectiveness of the rank 1 ability it is now...Nerfs shouldn't be that drastic. It creates a situation where champions constantly flux between being overpowered or underpowered, when a more manageable nerf can get the actual results you want. If it's still over the top shave off the power slightly more with a hotfix ~2 weeks into the patch.
My suggestion...Start off with a ~25% nerf to the base damage at all ranks so going from 80/110/140/170/200 to 60/80/100/120/140. If E is still too strong after (minimum 2 weeks on live) then shave off maybe 5-10 damage off each rank and see where it stand from there.
Gangplank's new kit is CENTERED around his barrels. Don't gut his main ability! It's already been nerfed in the previous patch making the ability harder to use, lower damage, and easier to counter. Without even seeing the effects of that nerf there are already more nerfs popping up on the PBE.