The WoW raiding experience, but without the MMO, and possibly the addition of rogue lite elements (each raid is a run with its own progression, but wiping would be allowed and embraced).

The format of DRG or Gunfire Reborn is pretty close, but 1) I prefer the high fantasy setting of warcraft to the gunplay, 2) I'm not interested in procedural levels, and 3) I want the focus to be on polished boss mechanics.

Dungeon Defenders is also close, but 1) you're defending instead of delving, and 2) it is also focused on killing waves of trash mobs rather than boss mechanics.

Destiny bosses are sometimes well designed, but 1) don't care for the gunplay, 2) classes hardly matter, 3) it's a max of 6 people, and I think closer to 10 is the sweet spot.

Gauntlet from a few years back was probably the closest, but still far from the mark. It could have used more mechanic heavy bosses, more meaningful gear, and a larger party size.