Something like an Armor/ManaReg item would be nice

Those aren't mana regen though, they're max mana. You'll still run out if you stay in the field long enough.

Not that that's bad necessarily.

What an armor/mana regen item (Seeker's + Idol or something like that) would be REALLY good for is mages laning against AD. Opinions may be divided on whether they need the help though. As a class, that is; I'm sure there are some individual mages that need some help, but an item anyone can buy may not be the most balanced way to provide it.

There's nothing stopping you from building Seeker's (or Chainmail, if you don't care about AP) and any of anyway, even though they don't combine with each other, and then finishing them in whichever order you like, if those are the stats that matter most to you.

P.S. Tanks don't usually need to build mana regen anyway, because their kits aren't that mana hungry and they only use their full combo in an all in, after which someone is probably backing or dying.

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Tanks and mages aren't supposed to share items. That is the reason zhonyas is their only armor item and abyssal is the only real grail

also tanks and ad champs don't really have items that work for both but they still use them. dead mans plate or any tank items. I've build diana with a dead mans plate. what i think is that riot doesn't want a great balance of tankiness and good offensive stats because it would be broken. thats why you don't have mana regen armor items because most people would rush them early.