Some kind of Battle Arena/Colosseum where players in preformed groups can enter once a day or week or something, and have to fight through randomly generated bosses, like a boss rush. It would be similar to Roulette:Trials, except instead of 1 fight, it's a series of 5. HP value and damage output are adjusted and start off easy but then ramp up as you progress, with the final boss being as hard as the highest current content level. Every fight is adjusted for level 60, so that even if the first wave is something like Ifrit, the mechanics are the same as Ifrit EX, but the HP is a bit higher so it's not facerolled by i210 in 4 seconds.

Example:

Enter the Colosseum

First Boss: Shiva
Second Boss: Oppressor x1
Third Boss: T11
Fourth Boss: Titan
Fifth Boss: Gaius Van Baelsar

a second time you enter, the lineup might be:

First Boss: T13
Second Boss: Ravana
Third Boss: Leviathan
Fourth Boss: Amon
Fifth Boss: Angra Mainyu

Mechanics that require 24 people from CT raids would be scaled down to work with 8, and even though you are given 1hr to win, it comes down to ~11-12 min per boss, so it's really a full fight , though the first few should be a bit easier than the last few