So I've been on a KASS-uh-Dhin binge lately. Enjoying him primarily out of mid. Biggest reason I'm enjoying him is that I believe he's a decent counter to the current lethality stuff.
I got very interested after I number 1 rated (per lolskill) Kassadin player from NA made this post and discussed his build/playstyle (I thought he was a challenger player but he's actually D2 atm). https://www.reddit.com/r/summonerschool/comments/5rzcwr/kassadin_explained_indepth_kassadin_main_1_na/
Anyway, I tried out his build and worked through the concept and I get what he's out to do. Starting there, then I started to watch current high elo games/streams from challenger level players to see what they do and why... which led me to a few vids from voyboy/valkrin. Voy plays much more assassin oriented (rarely if ever rod of ages) but gets the Hextech-glp800 pretty much as core. That's something I'm doing as well.
Here's voy's op.gg in case you are interested and want to see what he's doing: https://na.op.gg/summoner/userName=voyboy
Here's a few full games by voy
Last - here's a vid valkrin made that covers a more tank build and his reasoning. 10 min vid