Myself and a bunch others i know have reached the point where there are no more upgrades gearwise, to get.
the idea would be like this:
Normal-mode raid drops actual items etc.
Heroic & Mythic drops tokens to upgrade specific gearpieces by +2 / +4 Itemlvl.
Mythic + weekly drops gear and has an optional choice to get 3x+4 itemlvl tokens.
To prevent one from only upgrading specific pieces to +500 itemlvl, there would be a cap that prevented you from upgrading gear past +8/+10 itemlvls of your lowest gearpiece (equipped).
If looting and upgrading worked in this fashion, we'd have people running "old content" even though they had cleared all content.
To prevent previous itemlvl pieces to be relevant into new raids/seasons, the gear would gradually lose it's power by the release of new content.