So the setting would be a castle. Here it could be any Ishgardian castle. For now, the idea is 8, maybe 10 players. You start on the walls with the usual thrash, which will be called Map 1, and as you fail or succeed, you're forced to retreat to different places/having repelled the thrash assault go on to another place thats under siege, respectively map 2.1 and 2.2 from now. From 2.1 and 2.2 you'll be able to choose different paths, with different bosses and different mechanics, and maybe different loot, or just dropping a token which you can redeem for gear of your choice (Maybe redeemable tokens in NM, and real gear in HM). Possibly you can make 4 levels of gear, with 1. Loot from bosses from map 2.1 NM, 2. loot from bosses map 2.2 NM, 3. loot from weakest bosses map 2.1 HM and 4. loot from bosses map 2.2 HM.
The different bosses will require certain classes, for a certain point. An example would be running from a boulder that'll crush you, where you need the Bard's speed buff, cause you're TP is constantly drained due to a debuff. This is NOT gambling, when entering a dungeon with a certain party, as small hints will be shown, that give the party an idea what class is needed for that path (a picture of a music note implying bard songs next to an entrance).
The dungeon itself would be mediocre hard, it gives a challenge, and yes you might wipe, but its still easier than the hardest hardcore content, because lets be honest, even a dungeon with this replayability isn't fun when you clear all paths in 2-3 weeks. REMEMBER: this instance is less about dodging circles, beams or clearing dps checks, and more about a fun and innovative way to make certain classes needed in certain dungeons.