right now, the only thing to do with old gear (green/blue) is to turn it in for GC seals. There aren't that many useful things to buy w/ GC seals at the moment. Ventures, manuals for leveling DoH/DoL, and some vanity gear, but mostly garbage, and with all the extra gear we get from running old dungeons, I'm up to my ears in ventures and seals.
My solution solves 2 issues. 1) it gives us something to do with the gear that isn't useful any more. and 2) if gives us a reason to run older content.
Spitball idea: Let us convert the old gear into a piece of materia or token that allows you to upgrade a piece of gear by a level or 2. Nothing major, just something that gives you a bit of a boost and something to do with your gears.
Example: I am currently sitting at i175ish, with mostly 170 and some 180 gear. I have a full set of poetics gear for several jobs, and several partial sets of dreadwurm, high allagan, allagan, primal weapons & accessories, Hero's gear, Darklight gear, Boagatyr's, Gloam, Demons (WoD), Scylla's (CT), etc, etc. If I had the ability to take a set of left side ironworks gear, and convert it to a token, that I could use to upgrade a piece on my left side. And a piece of right side ironworks accessories and upgrade a piece of my right side, I could upgrade a piece from 180 to 182. This could be done by raising the base stats a bit, or by adding in a missing stat. So for my BLM, my i180 Asuran Dogi of Casting, has Vit, Int, Acc, and SpS. Upgrading to 182 could add a few stats to each of these, or even add a 5th stat (Crit, Det, etc).
Now there is a use for people to run older content. I'd go back and finish up those sets to get gear more often and use them to upgrade and help bridge the gap.
That or use them in place of the centurio seal upgrades, so that people aren't swarming the hunts so badly.
Just a though.