Theorising AP Itemisation Effect on Mage Itemisation
Current situation: If you're a Mage, you will pick one of the following items as a first item:
Generally, #1 is the most popular, with mages who support it building #4. Mages versus assassins or who focus on the lategame build #3. Mages losing against AP build #2. If you're Anivia, Ryze, Zilean or Cassio, you might build 
.
Beyond #1 and #2, there are no viable Mana regeneration items for most mages. The other Mana Regeneration items are generally considered good for Supports but not Mages because of the poor AP they offer.
New situation: Abyssal Sceptre loses early game power, so that's less of a pick-up when you look to do more damage. Its MR is better, but its AP is worse, pushing it into a space where damage-oriented mages have no use for Abyssal.
Athene's Unholy Grail becomes a Support item, or for the five or so Mages who can use it for when they lose lane. I want to say they'll probably buff Talon and Zed so this item sees more use. Either way, it's designed so that it's not an item for mages anymore. Xypherous said about Chalice that they're nerfing it to address bot lane sustain. Mages don't need options.
Rod of Ages gets a worse build path (Needlessly Large Rod instead of Blasting Wand), as well as losing out on AP and Mana. You gain Mana equal to 15% of damage received, gain Health equal to 15% of Mana spent. Champions who can spam their spells win in sustain but lose out in raw power. They also get to spam less because Mana for the item is nerfed by 300.
Tear of the Goddess is shoved into a better place for sustained combat (Mana cost down), directly translating to 25% more spellslinging.
Hextech Revolver becomes really good on AA champions, making 
excited as she now has even more burst to win her laning. Hextech also builds into more CC or a Dash, which further excites since it's now easier for her to land her combo on immobile champions. (unless they build Hextech Rocket Belt as well)
So in general, Mages will pick up the following as a first item:
Archangel gains new synergy with Morellonomicon, but it's not good enough to be built as a first item still.
Athene's is remade into a Support item and there is no longer a reason for mid laners to want to pick it up.
Rod of Ages shifts from an item for Mages who want to scale, like Kassadin, Anivia, Karthus, to an item mages pick up for bruising. I'd imagine it'll shift popularity from lategame scalers to midgame scalers, and from DPS mages to burst bruisers. Its synergy with Archangel's is disjointed.
Morellonomicon is the only 20% CDR item Mages have left.
Hextech Revolver becomes a good item for mages who use their AAs (not Karthus), and I'd imagine numerous would commence picking it up for the extra harass when brawling, at the cost of not getting CDR.
Prediction: I think Archangel's Staff will rise in popularity now that Morellonomicon becomes the only 20% CDR item for Mages, and it gives 400 Mana. Because Mana Regeneration is removed from Mages in its entiriety, and 400 Mana will not cut lategame siege situations, Mages will be forced to pick up Tear and build it out, shifting viable mages to those who benefit best as mid- to lategame carries, or those who burst/have innate Mana sustain.
Hextech Revolver empowers mages with serviceable AAs or AAs as part of their kits.
Clear winners of new itemisation:
These Mages all have burst patterns that get complemented by the new Hextech line, while having either innate Mana sustain (Annie Q, Twisted Fate Blue Card) or little to no siege ability so they'd want to use Mana to teamfight anyway. Orianna could be here, but I think her loss of sustained combat through no Mana Regeneration harms her a bit too much. Lux could be harmed by the loss of Mana regeneration, but I think the gains in laning phase burst pattern makes sure she still stays in the game. LeBlanc obviously enjoys the additional burst, Lissandra, TF, Annie have innate Mana sustain. Ahri enjoys the additional burst, but I'm not entirely sure she'll appreciate the lack of Mana sustain. All five of these last ones are happy Abyssal Sceptre will give them the 40% CDR they're looking for now.
Clear losers:
These mages excel at siege situations, or otherwise use large amounts of Mana they hope to recover with Mana Regeneration. That's being taken away with them. Anivia can't afford to use her legendary waveclear because it'd leave her without the Mana she needs to sustain teamfights. Karthus can no longer use his E for waveclear, and has to use its Passive to sustain his Mana instead. The amount of Mana he needs to use to clear the waves isn't feasible as he can't easily regenerate it anymore. Veigar's Passive stops synergising with popular Mana Restoration, but there may be hope if the little guy's Passive works with new Archangel. He can't throw E and W out anymore to siege and zone people easily, as he needs his Mana for teamfights.
Vel'Koz, Xerath and Ziggs are in the same painful spot of being artillery in an item system that doesn't support artillery mages.
Closing Note: Depending on the changes to the mages themselves, these item changes might not be that disastrous. We do have to hand in some AP from our end builds as Zhonya is further pushed away from AP mages. Itemisation supports AP bruisers better, and encourage mages using AAs to win. Itemisation also encourages taking the Mana Regeneration mastery over the increased damage, forcing AP Mages to make a decision between Sustained Combat and Damage.
Morellonomicon changes would mean Archangel becomes part of more Mages's builds, since they now all get 400 Mana out of a core CDR item.
Because Zhonya's been nerfed, it becomes less desirable as a pick-up even as it now offers CDR. Abyssal Sceptre users gain power since they now also get 10% CDR to round out CDR builds, whereas mages who excel at long-range are hosed.
Feel free to correct me where I'm wrong. I'd love to have a good talk on future itemisation for us.
