3 major factors many are overlooking in the mage rework

1. The meditation mastery

You gain 1.5% of your missing mana back every 5 seconds.

in seasons before building mana items was definately a bad idea on many champs who preferred mana regen but this mastery is the difference in s6

A dorans ring gives 50% extra mana. On a champ with say 8 base mana regen that adds 4 and it becomes 12. To get 4 regen out of the meditation mastery you just divide 4 by .015 to see how much missing mana you need

4/.015= 266.666666667 or 267 mana. A gives you 250 mana.

to replace the 100% on morellonomicon or athenes you'd need to replace 4 with whatever your base regen is.

Insert x/.015 into google for an interactive chart you can use to see pretty much every amount of missing mana you'd need to get a certain level of regen but dont forget you also go to lane with more mana

2. new item

   Lost Chapter (NEW)

    Recipe: Sapphire Crystal + Amplifying Tome + 115 Gold = 900 Total Gold
    25 Ability Power
    250 Mana
    Unique Passive: Upon levelling up, restores 20% of your maximum mana over 3 seconds. 
   

builds into morellonomicon which now increases max mana which now scales harder with archangel staff which has always had huge synergy with deathcap which also has synergy with lichbane which also has synergy with archangel's.

But even lost chapter on its own is very probably going to be enough for not-bad players on many champs. But the best part is what its replacing the mana function of, forbidden idol.

While the new price and whatnot may hurt a bit it also hurts the opponent both by limiting their options against your favorite pick and providing a cheap maybe or maybe not efficient way to get both mana and ap on a single item without getting ANOTHER dorans ring.

3. tanky AP champs like are getting a new buff out of the rod of ages.

prediction, jungle mumu is finally getting more nerfs, not that he's really that bad but his winrate has been consistently in the top 10 for years now just saying

also the new catalyst is a huge buff to AP champs that can regain mana I think especially with tear now giving mana back.

JUST SAYING