Honestly, they could swap Kog and Vayne's W damage types
Think about it. Kog's W does %hp magic damage, and yet he's the squishiest motherfucker around and is about as mobile as a disabled frog. Vayne gets %hp true damage when she already has a bunch of steroids and incredible mobility.
I don't think making Kog's W true damage would actually break him much (provided you removed the AP ratio), because he still has the big weakness of being totally immobile and his W has a downtime. Meanwhile, making Vayne's W do physical or magic damage would still let her hurt tanks a lot, but she'd be a hell of a lot less absurd.
#Truedamagekoggypls
For example. If he did "eh" damage to a squishy champion that just invested in Tabi and was slightly ahead whilst he is slightly behind, he's completly useless at that point with no real end-game other than maybe hide in the jungle/top lane and AFK farm until he's at all useful.
though - things like 
do hit her damage really hard, but if a Lux is doing half the damage she needs to - she can easily remedy that by just throwing out twice as many spells from a really safe back-line position.
- Kog'Maws substantial range, AoE, and scouting potential allow him to be a solid asset to his team even if he's behind. Throw hybrid damage and auto-scaling damage (%HP) in the mix and he's still pumping out some solid hurt when lacking a few items, especially as killing him doesn't remove his damage threat (and he does do true damage with his passive).
can poke her to death, and pretty much anything you don't want to get hit by, is within range to get hit by.
Vayne's also completely useless if she can't deal meaningful damage to someone. She has no scouting tools, no real poke (at least in a pre team-fight sense), no wave-clear. All she's got is 1v1 potential and damages.
