As a mage, I like the idea of unique pet items. I do not raid very often, so any method for accessing the opus tomes would be beneficial. Because the running out of expable items wall is half way closer to me, than a more raid inclined characters.
I agree with Dimmi, in the sense that whatever it is, it needs to be consumable, because without that the wall just gets pushed farther back.
Now for my unique idea.
How about a special zone(s), that is difficult and yields improved gear. But, to access it every member has to pay an exp fee to enter it. So, that way there is an incentive to enter the zone, but if you are bad or have unfortunate luck you must respend the exp to enter again. The only problem with it I see, are people camping ressers in the zone to help surpass that problem of repaying the exp after a wipe. You could even make it an instanced like HFB, so people don't enter and never leave waiting on respawns.
In theory, multiple difficulties of zones could be done, so that a person who just finished codices or about to start would be inclined to join in, but would slow their progression to reaching the end, while giving them more options and possibly gear.
Now for othe problems I see, to my idea.
If you had an easier version, people would bring in ringer characters, and just farm the instances to gear up alts. Which would allow for alts to be geared up much faster and successfully, then setting up a pug, or back gearing raid, with loot claimed with the same ringers. (That claim is my personal speculation, that it is easier to get 6 people than 18 in on an effort)
The Breakthrough effect: Initially, the new zone would be challenging and work very well at giving people an object to spend exp on, and challenge. After it was figured out, it will become easier and won't work as well as a sink, because people will need to buy fewer "tickets" when every trip is successful. Or contrariwise, the zone may be ignored as in Nadox. Though, I like that zone, but the only people I know who go there are Rolla and Tao. I never seem to be on when they do.
Mercantilism: A person or team may figure out the zone very well, and could easily 4 man it. By knowing that skill they could sell the other 2 positions for a chance at the loot they obviously do not need. This would be very contrary to the spirit of the game. I'm thinking of how when Meso would sell spots in an exp band, because he would get you an AA a day or something.