Overall, I think that in terms of mana, champions should be made like this:
--Champions that need huge amounts of mana for a massive extended combo, so they want as much mana as possible to unleash all at once. Their skills use relatively low amounts but cannot really be spammed. These champions have pretty big mana base and mana base per level, but relatively weak base regen and regen per level.
--Champions that constantly spam spells; as soon as CD is up, spell is launched in the general direction of the opponents. Spells may not have the greatest costs, but the low CD mean that if they don't have the regen, they will be dry. Moderate mana, but generally higher regen. Functions best with percentage based regen, but certain percentage based passives (or CDR runes) mean CDR+ManaRegen item isn't as needed, so a Bank+Regen item or pure Regen item can be used.
--Champions that are neither. For the most part these champions benefit from regen and CDR, but have a pretty good punch on their own not to be constantly on CD. The "Sniper" type mage, if you will. So will benefit from a general/flat mana/mana regen boost.
As for items. I think you should be able to do this for a basic/cheap combined item:
--Mana + Mana or Mana + Defense. So Catalyst/Glacial Shroud type items. A Mana + MR item should exist.
--Mana + Regen. Tear of the Goddess.
--Regen + Defense. Besides Chalice, something should build from both the Health Regen item and Mana Regen item that focuses on being a hyper sustain item (with obvious issues that it provides virtually no offense or defense). Maybe this item could be a new Mikaels (using its active to heal + give mana at the cost of nullifying its passive regens)
--Chalice should be harping on the fact it adds MR. So the MR from Chalice should feel pretty significant. The downside is that in all its full items, a key part is that it provides regen and MR (or Armor for its AD counterpart), so it's obviously going to lack AP and maybe CDR. The Athenes could also have its CDR reduced to 10% for a bit more MR and AP. Dunno.
--Mana + Health is fine.
Most mages should be building some mana or mana regen, but if you're really going at it because no matter what type of mage you are, somehow you're always spamming spells, then you trade DOT and or punch for more spells. That is obvious. Athene's/Morell should be hybrid items, giving a moderate amount of AP, some mana/mana regen, and some utility. If you opt for more mana regen (and to a lesser extent CDR), you get less utility (so from some ... to none), or less damage output (lower AP on already kind of weak AP items).