As many have noticed vertical progression in MMOs has gotten stale over time and every new raid is just boost to the numbers which in turn turns the older raids pointless so the carrot on the stick has gotten rotten after some time. Before big part of raiding , lets say 90% was preparation and the rest 10% was the execution which made the hard content last for long and killing the last boss after months made it feel like you accomplished something, now most of that has been "streamlined" and the RP in your G has been removed.
Even though vertical progression is flawed it gives the incentive to work towards something.
So i suggest that a system of vertical progression, not by armor, weapons or trinkets but with the mastery system and i dare say collections ( even though some are bad) and something similar to the PVP build panel, but for mastery buffs and skills only ****usable in this one raid and not anywhere else.** So lets say you have a boss with freezing powers and what not so your raid build consist of masteries that help you overcome wipe attacks, maybe you have a collection, which requires you to do fractals ,some old raid bosses,some open world stuff, some crafting and then it opens a mastery that lets you craft a cauldron of thawing elixir like the one in Bitterfrost Frontier which gives you a buff reducing cold damage of the area. Even lets say you get a mastery that breaks a wall and opens a bonus boss and what not. As you see this is somewhat influenced by the way SAB works and i wonder why the devs didn't use similar design in the rest of the game.
Maybe this should be only Challenge Mode system and raids stay as is so it doesn't alienate the community. Also creating this for CM as a long term goal might spark some competition like in Warcraft(even though it's finished in 3 days as opposed to before when it took months), people seem to care who will get the world first when its presented like a race. A leaderboard for who killed each boss first, who has the fastest full wing clear or boss would also help.