AP Fighter Solution (Idea)
Who Are AP Fighters?
Tank/Fighter
Juggernaut
Diver
Diver
Mage/Tank
Mage/Tank
Juggernaut
Tank/Fighter
Fighter/Mage
Juggernaut
Juggernaut
Diver
Juggernaut
Creating a Stat for AP Fighters
Ability Power in general needs more stats to scale with since currently it is only Ability Power, Magic Penetration, and Cooldown Reduction. Attack Damage on the other hand has Base Attack Damage, Bonus Attack Damage, Attack Speed, Armor Penetration, Critical Strike Chance, Critical Strike Damage, and Cooldown Reduction. Since Attack Damage based champions have a lot more options to scale with it is much easier to have many successful roles for Attack Damage based champions. A proposed idea that I have seen is that Riot could add base Ability Power to champions. I think that this could be the solution. I believe that all Riot would have to do is give the Champions that they define as worthy of being able to build AP Fighter the stat and so only the champions I listed above would be granted the Base AP stat. This is a way so that Tanks, Assassins, and Mages could not abuse AP Fighter items.
How Base Ability Power Could Look
I think it could work just like Attack Damage, but only be included on champions that are intended to be AP Fighter like. So each champion would start with a certain value and then gain some per level. This would cause a few problems IMO because there are so many champions that would not have any AP at the start of the game except through runes making it harder the balance early items because champions may abuse early mage items if they have good starting base AP.
I think that one way to do it would be to have all AP Fighter like champions start with 65 Base AP and get 5 AP per level totaling to 150 Base AP at level 18. I think with this method it would be much easier to implement items and new scaling to the class if the numbers were consistent. If it was treated like how current Attack Damage works I think it would take way too long to figure it out since so much has been but into creating balance with the rest of the roles in league and treating the same as Attack Damage would break too many champions because of items.
AP Fighter Scalings
So since AP Fighters should scale with different stats than AP Mages, so changing mages scaling to Bonus AP which is just a clarity change since they don't have Base AP and AP Fighters scale with Total AP. Also AP Fighters should scale with Health. Also this is conceptually more hard to explain than the rest of my post, so I will give an example.
Example:
Diana:
Passive - Damage 20-250 + (80% AP) > 10-300 + (60% Total AP)
Q - Damage 60/95/130/165/200 + (70% AP) > 50/80/110/140/170 + (60% Total AP)
W - Cooldown 10 at all ranks > 10/9.5/9/8.5/8 Damage 22/34/46/58/70 + (20% AP) > 30/40/50/60/70 + (20% Total AP) Shield 40/55/70/85/100 + (30% AP) > 0/15/30/45/60 + (15% Total Health)
E - Cooldown 26/24/22/20/18 > 20/19/18/17/16
R - Damage 100/160/220 + (60% AP) > 150/200/250 + (40% Total AP)
Something along the lines of that. Also adding Percent health damage that scales with AP like 1% per 100 AP would also help Fighters.
Theoretical Items
(These items are based around the idea that every AP Fighter starts at 65 AP and scale up to 150 AP as base AP)
Name: Hextech Omnisythe
Build: Hextech Revolver + Crystalis of Aeons + Kircheis Shard + 583 gold = 3333 gold
Stats: AP 50 Health 300 Mana 300 Attack Speed 25% Cooldown Reduction 10%
Passive: Magic Bolt (same unique passive as Lighting Bolt) After casting a spell your next basic attack will deal your base AP as bonus magic damage. (1.5 second cooldown)
UNIQUE – ETERNITY: Restore mana equal to 15% of damage taken from champions. Restore health equal to 20% of mana spent, up to 15 health per cast, while toggle abilities can heal for up to 15 per second.
UNIQUE – ENERGIZED: Moving and basic attacking generates Energize stacks, up to 100. Once energized your next basic attack will slow for 40% decaying over 1 second.
Name: Gunther's Untattered Cloak (Melee only)
Build: Fiendish Codex + Negatron Cloak + Amplifying Tome + 945 gold = 3000 hold
Stats: AP 60 Magic Resist 50 Cooldown Reduction 10%
UNIQUE: Nearby enemy champions have their Magic Resist reduced by 15% of your Base AP (300 range)
Name: Divine Eclipse
Build: Seeker's Armguard + Giant’s Belt + 900 gold = 3100
Stats: AP 45 Armor 45 Health 400
Passive: After receiving 90% of your base Health(within 10 seconds) in damage receive a shield for 300% of your base AP.
Edit: Forgot to add that Deathcap would have to have it's passive switched to be increase Bonus Ability Power instead of just increasing Ability Power.