Kappadin is perfectly balanced

http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/1868202782/50734325?tab=overview

No but seriously even though I do think he sometimes is underwhelming, he still had the snowball effect on his kit... IMO they need to move a lot of damage to his mobility, reduce it's aoe radius at which it does the damage, and increase the base CD to 4 or 5. Having Riftwalk every 1.6 seconds is pretty overwhelming, even at 500 range.

TL:DR make Riftwalk base damage 150/200/250 + 0.7 AP at 550 range in a 100 circle radius, and make it's CD 4 seconds at max rank, reduce Q damage to 140 + 0.5AP, E damage to 160 + 0.7AP