Mages can build AA Staff or Morello's for mana regen.
Let Tanks have Athene's.
New build: 
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Mana makes the passive more effective. All-around defensive stats makes it a great core item for all those mana-based utility Tanks since sometimes Frozen Heart just isn't a great buy. And of course 20% CDR for extra spell spamming. Solves the overlap issue with Morello's and gives Tanks a second 20% CDR option.
KIei2 votesTanks focus more on health rather than mana regen.
Leti the Yeti1 votesI would argue that most tanks dont need mana regen?
Kratos2632 votesI would argue that most tanks dont need mana regen?
You may be mistaking bruisers for tanks. Even though some can now buy be built for more damage, here are some real tanks who uses mana: Maokai, Nautilus, Malphite, Sejuani, Amumu, Rammus, Galio. I don't know about rammus, but without blue, these guys have a really hard time keeping their mana up, and I've only seen galio and malphite using athenes, and when malphite buys it, he's usually playing ap instead of tank.
That's also one of the reasons most of them don't lane. They need their skills early game to stay alive and put pressure, but they just can't afford it, which is why most of the time you see them in lane, it will not be as tanks, but as ap bruisers mostly, and besides that one time with maokai, they aren't always that good at it.
Leti the Yeti1 votesOh hey, I approve of this aready.. If I had mroe mp as malphite top.. everyone would suffer
kay balance issues are gonna arise, cuz Malphite is really tuff if he is gonna have high mp regen
exiledFromBadmen1 votesI quite disagree with Sejuani. If you don't spam your Q, you won't be oom. Her mana costs are such low, and her base mana is doubled.
But, Blitzcrank really needs this item.
WARLAG01 votesThat would be cool, seems really beneficial, but I'm a Galio main so I could be somewhat biased as this would be really good on him. Anyways, if this doesn't happen (and I think it won't) at least buff up Athene's. I was mid laning with Galio and around lvl 9 or 10 I randomly looked at my mana and thought "what the hell? I got less than half mana? I'm not even spamming too hard." Maybe I was spamming, but it didn't feel like before when you could really spam abilities that's for sure.
OJ1911 votesyeah how about no
I'd rather not have blitz or malphite spamming shit all day
Rebonack1 votesyeah how about no
I'd rather not have blitz or malphite spamming shit all day
But Ori and Syndra spamming shit all day is totally okay.
That Mage bias is too real.
Anywho, as was so kindly pointed out there are plenty of real Tanks that have mana issues if they try jungling without Blue. Amumu and Malphite are both notorious for it, in fact. Malphite was known as 'The Rock with no mana' back in the day for a reason. It's the various Fighter/Tanks who don't give a crap about mana regen because their mana costs are completely negligible.
OJ1911 votesBut Ori and Syndra spamming shit all day is totally okay.
That Mage bias is too real.
Anywho, as was so kindly pointed out there are plenty of real Tanks that have mana issues if they try jungling without Blue. Amumu and Malphite are both notorious for it, in fact. Malphite was known as 'The Rock with no mana' back in the day for a reason. It's the various Fighter/Tanks who don't give a crap about mana regen because their mana costs are completely negligible.
Sigh yeah syndra and Ori need nerfs, but we weren't talking about them.
There are valid reasons amumu and malphite aren't allowed to spam forever by the way, especially malphite who if/when he can, likes to sit top lane and spam point and click damaging slows at you all day.