Hybrid Varus
Almost all of the builds/guides I see for Varus build him as straight AD, or with a Trinity Force as the only AP source, but looking at his kit I think he could really benefit from a hybrid build thanks to his W passive and AP scaling on his ult (which only has a 70 second cooldown). The % of max health in particular could really add up considering it is per stack and all of his active abilities have the potential to hit multiple champions.
I have some thoughts for a hybrid build, but also some questions, namely, how will Spellvamp treat damage from his W passive? If it works with it as single target, then it is a nice little boost, other wise not worth it.
Things to include in build: Trinity Force (best Hybrid item) Infinity Edge (still an ADC) Hextech Gunblade (?) Nashor's Tooth - (CDR is important for that W passive, getting off another round of abilities is huge) Berserker's Greaves Defensive Item
Another possible consideration would be Rageblade, particularly if losing. I'd say Essence Reaver, but I don't think he's that mana hungry. Maybe Wits End as an option instead of Nashor's Tooth.
Mostly just spitballing ideas, here. Would love to hear from someone who knows better.