I noticed there are 3 tenacity items.
Mercury Treads (boots)
Zephyr (AD/Melee based item)
Spirit of the Ancient Golem (Tank/Jungle based item)

There should be an AP item with tenacity. There was item in Season 2 that had like 50 AP and tenacity but no one used it becuase it was not very good end game. I would suggest a Needlessly Large Rod pay like an additional 800 to turn it into a tenacity (35%) item with about 90 AP. I would make an argument for support Tenacity item as well, but 1 thing at a time.

Am I the only one who thinks there should be an AP tenacity item? Please give some constructive criticism if I am wrong.

I agree, I tend to like ap mid champs, and hate being countered by hard CC because merc treads aren't a great caster item (I either get ionian boots, or sorc for cd or pen) and AP doesn't have a hard form of passively escaping from hard CC (besides QSS)

You know, there used to be one but no one ever got it so they removed it

Late game all AP casters should be swapping out their boots for merc treads anyway, there are more important AP items to buy than one that gives stats your boots can give you.

I miss Moonflair Spellblade. I bought it every game against Veigars and Kassadins (anyone with stun-central combos).