When the newest raid is released, the old gear is no longer useful and the content tied to it also isn't needed. There's also a lot of demand for recolored versions of the old raid gear for glamour, so here's my idea.
The newest raid gear, let's use the current levels as an example, comes out at 210. Now some of the stats on those pieces aren't good or are sometimes flat out terrible but we have very few options for gear since iLevel trumps (basically) all. So you either take a piece with better primary stats from the higher iLevel, or better secondary stats with a lower iLevel.
So my suggestion is that you get the newest raid gear from the newest raids and spiritbond it. Once the gear is spritbonded then it can be converted into an iLevel augmentation. For example, Alex Savage gear will become an「Augmentation: iLevel 210」. Now you can use this augmentation to upgrade old raid gear to that iLevel. So if you like the stats of the High Allagan armour, using the augmentation allows you to upgrade it to an equivalent i210 piece.
For example, the i210 Gordian Body for BRD has Dex-101 Vit-107 SS-94 Det-63, but let's say I would prefer more critical-hit rate. I would do the following steps:
1. Get the Gordian body from A4S (bear with me non-raiders)
2. Spiritbond it
3. Convert it to an 「Augmentation: iLevel 210 (Body)」
4. Farm turn 9 for the High Allagan Coat of Aiming
5. Trade both into an NPC where I would receive an Augmented High Allagan of Aiming (i210)
The stats on the augmented piece would follow the weightings of the original piece with its stats increased from Dex-39 Vit-46 Crit-41 SS-29 to Dex-101 Vit-107 Crit-94 SS-63 to match the main and secondary stat levels of an i210 piece.
Now people are probably saying 'What about us non-raiders?', well you could do the same with tome gear, except first you would need to upgrade it from the i200 to i210 piece with other tokens as we are now, spiritbond the upgraded piece, then augment it. You could continue to upgrade the same piece of augmented armour using the flexible-stat equipment feature that they used in Diadem, or force people to go back and get a new piece to augment every time new raids are released.
If you wanted to make it more challenging to do, or encourage more people using duty finder on old raids, you could make the content also give you a catalyst which only drops in a level-sync raid so you can’t just blow through the content with your level 60 characters. It would also keep the content challenging after the character level increases again (probably) in the next expansion.
Just as a note, I’m not sure how materia would fit into this, depends a lot on how they implement it. Maybe we’ll find out more during the live letter. In the meantime, what do you think? (Did I explain myself well enough?) And questions?