If you don't want to read, skip to "Items to fix"
My take on a problem I have with casters
I personally feel, especially given the current tanky meta game, that mages are in a very difficult spot; especially late game. The main problem I have with them is that a mage needs to have an insane amount of gold over the course of the game to stack the very large AP items. There two reasons for this:
1) Many skills do not scale that great with AP on certain mages (around 0.5 - 0.7 AP ratio)
2) Stacking MR is fairly simple to do. Given the tanky meta, having 80-100 MR is very feasible.
However, in general, this AP is hard to get because mages are squishy (by design of course) and in general have little defense, except possibly health from a rylais, RoA or Haunting Guise. Having so little defense they are generally reasonably easy to shut down (or easier than other characters). This is, again, by design as they generally have such massive damage.
This is the catch 22, given the current tanky meta, mages don't really have that much damage to write home about unless they have laned perfectly, got early kills and never died. IMO there is very little room for error with a mage, one mistake can affect you more than other classes.
Items to fix
This plan would be a new item which will build into another item.
Item 1: Enchanted Mantle
Cost: 435 gold
Stats: 30 Armor
Item 2: Cloak of the Wizard
Recipe: Enchanted Mantle, Blasting Wand, Amplifying Tomb, 1000 gold (total is 2730)
Grants: 45 Armor, 70 AP
This would allow a mage, should they choose, to start with a bit more physical defense early on with the Enchanted Mantle. Considering how important AP is, for dedicated casters armor is usually somewhat of a luxury. This however, would allow for a reasonable amount of armor with a reasonable amount of AP. Note this grants no health or other bonuses.
The idea is it should, in theory, make their laning phase stronger with armor and allow them to hopefully farm up their much needed gold easier.
On champions like fiddlesticks, this would allow his ultimate to see somewhat more use as the physical carries would have a more difficult time bursting him down. Note that should fiddles still ult into a team alone, 45 armor is not going to save him from the insane burst he will receive and will still melt.
Nunu is another champion. Due to his tanky nature (he's even labeled with tank) would help him fill that role while still giving his spells some kick.
Possible problem champions
The first to come to mind, and I'm sure you all thought of this, is Cho'Gath. Cho doesn't have HP problems, giving him AP and a good amount of Armor in one go seems to be just too much for him in particular.
That's my lot for this, I hope people can offer constructive criticism.