In all my games, almost trying all champions, I realized that there is no AP based on HEALTH item.
Archangel Staff. (AP/MANA)
+400 Mana +25 Mana Regen per 5 seconds +45 Ability Power
Passive: 3% of your max Mana is converted to Ability Power. UNIQUE Passive: Each time your champion uses an ability their max Mana will increase by 4 (3 second cooldown). Bonus caps at +1000 Mana.
+350 Mana +7 Mana Regen per 5 seconds +20 Attack Damage
UNIQUE Passive: Grants Attack Damage equal to 2% of your maximum Mana. UNIQUE Passive: Each time you basic attack, your maximum Mana increases by 1 (3 second cooldown). Each time you use an ability, your maximum Mana increases by 4 (3 second cooldown). Bonus caps at +1000 Mana.
Atma's Impaler. (AD/HEALTH)
+45 Armor +18% Critical Strike Chance
UNIQUE Passive: Gain Attack Damage equal to 1.5% of your maximum Health.
After taking a look at this items, I think:
It would be great to AP chars to have an item that gives you AP based on health points!
Think of that, champions like nunu, akali, katarina, mordekaiser,(almost any AP champion) could take a lot of advantage of that kind of item.
And that's my personal opinion, I hope you considerer it and create a new great item!
Pd. Im tired of buying always the same items, instead of create more champions, you could create more items to different builds of any champion individualy.