I have an idea that might work.
We have 12-13 gear slots:
- weapon
- head
- body
- hands
- waist
- legs
- feet
- neck
- ear
- wrist
- ring
- ring
- shield
You could have each raid buff only 4 of the slots instead of all 12 or 13. Each turn could buff a piece (Turn 1: head, Turn 2: hands, etc.) Or you could get multiple upgrades for a single turn (Turn 1: head, hands, wrist, ring, etc.) In the latter case, the later turns would offer additional upgrades. So you could buff hands 4x, head 4x, etc. This would preserve the extreme vertical progression for the people who are really into that but still allow people to keep their BiS pieces for longer. Every raid would offer gear improvements but each BiS piece would be good for roughly 6 patches (about a year and a half).
My solution is very basic and really not that imaginative. I'm sure game developers could come up with something even better. Regardless, I think they should do something different. Gear becomes obsolete too fast. Raiders don't get to enjoy having the best gear long enough to make raiding very motivating. I don't raid much now but I know I would raid more if the next cycle of tome gear wasn't going to be better than raid gear.
Edit: I just had another idea. You could also have gear that gave buffs to non-battle content (or could be traded in or used to augment other gear with non-combat buffs). Some examples: +MGP earned, increased Triple Triad drop rate, +Wolf Marks, Move - Find Item, increased movement speed (outside of combat), +crafting/gathering stats, +choco exp earned (regular or racing), and so on.
There's no limit to the amount of options that would be a lot more interesting than what we have. I like the idea of old gear being consumed to augment new gear with cool perks. It would also keep a steady supply of people doing older content (to add perks to their new gear)