First off, I'd really love for a Rioter to read this, because I have a legitimate question for anyone on the balance team or design team specifically. Have you guys ever considered trying to make AP fighters more realized as a class? I personally believe it would open up a lot of space in comps as currently top lane doesn't have a plethora of options for AP champions, and none of them happen to be bruisers/fighters in the traditional sense.
Anyways, some background. AP fighters in the game do *exist* but not in a way that actually fulfills the "fighter" archetype. The three main champions I think of when I think of an AP fighter are Kassadin, Akali, and Diana. Generally, fighters should be melee champions that aren't surprisingly bursty, have some cc in their kit but not a significant amount, and build hybrid damage/tank generally.
So, what are the problems with these champions? Well, each one has his/her own problem.
For Kassadin, he's only viable as a mid laner, and his build path is quite linear. This is because he's designed as an anti-AP champion. He's also quite bursty, but not so surprisingly bursty that I wouldn't classify him as a fighter. The real issue is that he's not *really* a melee champion in terms of what he actually does. Most of his damage comes from his ranged abilities, which is uncharacteristic of the class. You might not want to consider Kassadin a fighter, and that's fair. But he's fairly close on the spectrum.
Diana has one major problem; she's too bursty. I think Diana was originally intended as an AP fighter because of her kit specifically. She's a diving fighter with some innate tankiness in her kit, with some power in her auto attacks. Seems very fighter-like. The issue is primarily in her AP ratios, where her power is, and itemization (this is a major topic I'll address shortly).
Akali is probably the best example of an AP fighter in the game for a few reasons. Firstly, she's viable vs other fighters in the top lane, which is important. As well, she's not insanely bursty as an AP champion. Lastly, she does have defensive tools at her disposal. She also suffers from itemization, but to a lesser extent.
Aside from these individual champions, there are also some itemization concerns. If you look at AD fighting champions, they have a plethora of items to select from that cater to their playstyle. Triforce is so ubiquitous among some of the top lane fighters that many of them are balanced around the item. But these champions also have access to the Hydra items, which help splitting and make them beefier; they have access to sterak's gage, which is a phenomenal item for diving; and lastly they have GA. Another notable mention is that most of these champions can opt into full tank items, because they only need a few damage-oriented items to be effective as fighters.
AP items don't have equivalent options. Most AP items are dedicated to high AP ratios with a semi-unique passive. But most of them don't have any tank stats included. RoA is a notable exception, but having to build it as a first item feels awful as it heavily delays the power spikes of champions. If Sterak's was like this, it wouldn't be built. Otherwise, the only other decent option for an AP fighter is Lich bane because it encourages a diving playstyle.
Some propositions I might have to fix this: one option is to rework RoA to not be so linear. Sterak's Gage is a proven item that has been phenomenally successful, it might not be a bad idea to rework RoA into something like Sterak's, which would hurt some champions like Annie who often build it, but might benefit diving AP fighters like Kassadin and Diana. Otherwise, I just think new items need to exist as options for these champions. Whether it be some waveclear item with health, or perhaps something more unique.
If you read this entire post, good for ya. I understand that AP as a stat tends to be more bursty by nature of the fundamental design of the game, but I would still like to think that there is room for AP fighters in the game, given we create space for that role. Let me know if you disagree with what I'm saying, I'll even throw in a poll.