K so. Been playing with the Ryze rework, spectated a few games (and played a couple of my own), I have some possibly aimless banter about everyone's favorite tattooed blue man. What the heck is this new passive?
So the quick explanation of the new Arcane Mastery is "power spike with a build up" (think Renekton or Gnar). Ryze can't do his notorious machine-gun-spell slinging without a build-up first, but when he does, his bursts are much bigger than old Ryze since during this spike, W / E / ult all reset Overload's cooldown, making for much bigger bursts and bouts of DPS. We're talking up to 6-7 spells in a normal bout post-6, and 3-4 of them being Overload. It also makes Desperate Power's cooldown extremely low in teamfights.
The shield also makes for a great baiting tool, since you don't even have to hit your spells to set it off. Overload's huge mana cost reduction also helps a lot with this.
Sample build
I saw someone else on the PBE building Ryze this way. Spectating a game of his (and a few others), I noticed that the damage that he did with it was way higher in the late-game than builds that included more mana / tankiness (traditional Ryze stuff), which is what I and a couple others were doing.
That said, I tried out the build you see above and it's the real deal. Thanks to the AoE damage between Spell Flux and Desperate Power, any two targets who stood beside each other would get dropped almost instantly, with me healing up almost 2/3 my max health when all was said and done with that Spell Vamp (which will probably be nerfed before it goes live).
AP wasn't a staple with Ryze before...why now? Well since Overload actually has decent AP scaling now (40% => 55%), and you'll be hitting way more of them, don't be afraid to give a little more AP-ish build a shot. Of course, the main focus in your build should still be mana, although Frozen Heart probably won't see much use anymore due to the 30% cooldown reduction Ryze gets passively with a rank 3 Desperate Power, and masteries, runes or the blue buff will bring you to the cap.
Lich Bane? Yeah, it was a niche item on Ryze before, and it still seems to be one. But if you have the opportunity to weave autoattacks in your spells (as you have access to Lich Bane proc pretty much whenever you want it now) you may come to appreciate the extra damage it offfers. With the changes to Overload it makes a more viable item than before as, of course, it gives you a Sheen proc whenever you want one, that can be handy for sieges and even for splitpushing (Ryze isn't too bad at waveclearing without his ult now thanks to his new passive).
Is Ryze going to stay a top lane mage champion?
Dunno, but if I had to say yes or no, I'd say probably yeah. Overload's changing to a skillshot makes for a safer laning phase where Ryze can be free from oppressive upfront damage that some solo laners can dish out. Of course, he also remains a fairly solid pick against highly mobile top lane champions (Fizz, Riven, Hecarim, etc.), but then again that's an argument you could make for taking him to the mid lane as well, where there are things like Yasuo and Zed.
What will change in his playstyle?
Not a whole lot, really. Though Overload becoming a skillshot does bring safety in lane, it also can work against Ryze in teamfights as it can be meatshielded by a tank if he's trying to reach someone in the back line. That's where Desperate Power comes in though; grouped up enemies will take damage from each other, while Ryze has the movement to get around larger obstacles such as those pain-in-the-ass tanks.