Ok,so this is my first try at making a guide so I'm starting as humble as possible. Since it's a new champ, I thought it would be ok to give it a try with what I reckon will make you quite the annoying mid/top Viktor. It's just a draft but I hope it's useful.

Be nice to me :)

As item build goes, first thing to be adressed is what to do with your Hex Core. As you're going solo, the two more utility directed improvements seem more interesting, and of course they all are, but really, it depends on how you intend to build the rest of your items.
First one I tried was Power. Power is awesome if you feel you're being harassed too easily. Not only it gives a nice health boost early in the game, it also keeps you in lane longer with the health per 5. The small speed boost isn't to be taken lightly either, it can save you quite often, specially when you're soloing and vulnerable to ganks. However, as I said, it all depends on the rest of your build. So as far as health goes, I got it covered with my runes. For the speed boost, I chose to give him Swiftness in the masteries. I know its not the same, but having it from start helps a lot.
Second one I tried was Gravity. It's pretty awesome. The mana and mana regen really help at start, the cooldown reduction is great, but really, it's charm is that 30% range boost on gravity field. This helps in several occasions, from preparing a gank to simply harassing your opponent and coming out of it unscarred. Well as for mana and mana regen, The items got it completely covered, but that little gravity field boost is something that you should keep in mind to get depending on your play style.
Death. It didn't seem attractive at first. Actually it seemed one of the dumbest of all three ultil a friend told me to give it a try. Well yeah, it's AP and a damage over time of your top damagin ability but look at all the other nice things the others can do. However what I didn't have noticed is how easier it gets to farm from the start. Ok, not start, but as soon as you get your Boots of Speed. Really, it's quite the difference, not to mention that that little damage over time can make wonders to fleeing enemies.
Well that's it I guess, if it looks good I'll see what to do with this build. Again, hope it's useful.