That could be OP, unless it was gated by his mana.
The problem with AS fighters is that their pretty binary in their kits. If everything is on their auto attacks, they either do too much damage or not enough. And nothing else can really change that.
on second thought, That Would make his ability OP because of how his Ult works.
I tended to build him more Bruiser than assassin. Focusing on damage from one item, and then building tank. Splashing CD for ult engages.
If I was to play WW (perhaps on a dare) I think I would try Cinderhulk (cause OP), BotRK as my main damage Item, Perhaps a Righteous glory as an initiation tool. Theirs probably a few items that would work on him.