(I'm brazillian so I may commit some grammar mistakes in the text)
With the Guinsoo's rework, many champions stopped building it, while others considered doing so. But most people thought that building it wasn't the best choice considering the price and the necessity of stacking it and maintaining the stacks. And as you all know Varus disappeared after they removed flat pen from the game, the same happened with Jayce.
But, I'm a guy who's always trying some new metagames to kill the bore of 4 years of League Of Legends. And when the guinsoo's rework came I first thought with marksman champions who need to proc hits to do some damage like Vayne or Twitch. But those champions don't have the best AP scale in their skills. And with some more thought Varus cane to my mind, he's perfect and I'll explain why.

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EXPLANATION

First his passive gives him attack speed like guinsoo's so he won't have problem stacking it. And the main reason is the W. After you build Nashor+Guinsoo and have the AP runes, you'll have 50 total bonus damage in the W. Not only that, but at skill level 5, W's Blight will do 5+(3%) damage, only in this build. Which means, if you stack 3 Blights and use either your E or your Q to proc it you'll do 24% based on maximum health MAGIC damage. And putting 3 blight stacks won't be hard since you have all the attack speed and the Guinsoo's double hit, and it will be even easier when you build the Cannon. Having all this AP damage, but you'll still have some crit, and your Q's AD damage plus the BORK's AD health% DMG. So I won't enter in details about all Varus's kit but it works. And I promise you won't lose your time, try it on normal game or against bots, and after that you'll notice that this build is more than viable, it's overpowered. Not only that but the build CDR will allow to use your skills with more frequence, and have a 42 second cooldown in your ult at level 3, always use it when you can.

You'll use normal AP runes for this, but with some attack speed if you wan. You also might wanna do a rune page that will give you the (3%) on W's blight when nashor and guinsoo are built. Otherwise you'll lose 3% bonus AP damage.

You can use your normal ADC masteries with attacks speed and all, but you might want magic pen and thunderlord to maximize your damage.

Varus will need a lot of protection since he doesn't have mobility, but I recommend playing with a Braum since you'll proc Braum's passive easier and will have a nice protection.