SUGGESTION 1 (RAIDING). Keep the raiding gear the way it is right now for the three stages of the raid. When you finish the final 12.0 of the raid and have the max raiding gear before the next expansion do this. Make it so that you can turn the raiding gear from stage 1 and stage 2 into items to further upgrade your stage three gear respectively. This upgrade should add some beneficial effect that will always keep it relevant. For example, you can make it so that the fending Armour gets a 2% reduction to all damage. Or can give the weapons a +2% life-gain. Or for dps the final weapon upgrade can give a new instant ability on a 60 second timer that only works while it is equipped. That kind of thing. Then on future x.1, x.3, x.5 patches (or better placed), you can get items that will upgrade the top tier raiding gear from old expansion raids to have equivalent stats to the new on. This will still make the new gear better for half the cycle, but keep old gear relevant.

SUGGESTION 2 (EXTREMES). This kind of goes with what I just mentioned above. Perhaps make it so that the extremes drops (or token trade in) the items needed to upgrade old relic or raid gear to be equivalent to the new stuff. That ensures that extremes are always worth doing when they come out besides just a new weapon.

SUGGESTION 3 (RELIC). First of all, what I mentioned above could be substituted for the reward for the relic or even they both do it. This especially works since raiding is more skill based whereas relic is more time consuming based. Furthermore, if they start doing new content or better stories like they did with Eureka, then the relic is almost there. Essentially I think Relic gear should work the same way raiding gear does with the possible extreme drops to upgrade the stats to be relevant, but the nature of the additional unique feature should be different. For example, perhaps raid gear gives stat increasing (like the % life steal or permanent regen) whereas relic gear either gives cool down reduction to certain actions/abilities or gear equipped only instant abilities and such. Basically make either relic or raid passive upgrades whereas the other will be active upgrades.