Mage item update will ruin ARAMS
Without the ability to back, mp5 is essential for mages in ARAMS. Moving all mages to using flat mana will make them awful in ARAMS.
Even if you increased the global buff to mp5 to offset this nerf to mages, then Supports, AD Casters, and "champs that are gated by mana early game but since they aren't mages it's okay for them to effectively have infinite mana late game" (e.g. Darius) will be overpowered as a result.
I know you don't balance around ARAMS, but there's a difference between some champions being particularly strong on it, and an entire class of champions feeling horrible on it (and all required to take clarity to even function).
Please take ARAMS into account when you are investigating the AP itemization update.
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etc. who are near constant spellcasters are pretty annoying with copious amounts of mana regen/cdr in one item. I think the mage update will throw ARAM balance around a little bit, but all the champions I mentioned can also build Tear into Seraph's to allow for endless spell spam, or they can opt for the support item build (which is probably better anyway due to gold income). I think that the champions I mentioned will still be strong because they are really good at the ARAM format due to their kits. Lacking mana regen in ARAM isn't really a big deal anyway as you have increased mana regen on the map and the "health packs" give a good amount of mana as well.