I think the problem is that everyone wants DAMAGE, which for ADCs can be split between Penetration, CDR, AD, Crit Chance, and AS, but they also need MS. So they have a LOT of options.
For mages, you really just want tons of AP (that what everyone seems to focus on) so that leaves fewer options. I mean, sure, they want some mana and CDR, but that is easily corrected....
I think the biggest change recently was the removal of Locket and the addition of MR per level, which will allow Riot to explore more MPen options. They just havent had enough time to implement it yet.
I agree that Hextech Items were a step towards this, but RoA > alot of other options and there arent enough hybrid champions for Gunblade.
Id definitely have to say if diversity were to come in, it would be best in the form of Morellonomicon splits like during the Black Market Brawlers period.