A REALLY HARD DUNGEON? This would be a turn of course, and not the entire raid. Imagine T3, just make it hard. No boss at the end, but just a ridiculously hard dungeon (still lots of mechanics and such).

Personally I'd like a dungeon where a near invincible monster is chasing you and you have to clear mobs before he catches you. Imagine wanderer's palace, but with placing mobs in certain locations like placing the heads in Qarn normal. While dealing with monsters that can cause hysteria/stun/silence, monsters that can heal others, monsters that hit like trucks and might even require time swaps, monsters with large AoEs or DoTs, and traps.

Kill order and positioning will be important as well, and of course monsters will have really high stats in general to justify a party of eight doing it. Imagine this as T14 if binding coil were to continue, an t15 you fight the monster chasing you once it enters an area that weakens it or whatever.

I guess something like t4, but you would actually move forward and not just be stuck in an arena, but that's how hard the mobs would be and such (harder obviously with gear/level cap increase).