I'm hoping it's not:

• Content that is just the same as the dungeons we have right now, but scaled up to accommodate for >5 people. Stack-and-spank with 6+ people isn't challenging.

• Content that relies on a gimmick that requires coordination or logistics of greater than five people at a time. Guild Puzzles aren't challenging.

• Mechanics that are overtly unforgiving and require tremendous backtracking or instant-kill mechanics. This isn't challenging, this is frustrating. Dark Souls punishes you for mistakes, and is challenging. Contra frustrates you until you enter the Konami Code.

• Content that can be trivialized by correct timing of dodges or reflects.

• Endless horde defense. I get enough of that in Warframe's "endgame".

I hope it is:

• Randomness can be fun when used appropriately because it forces players to adapt on the fly. Too much randomness can ruin fun, but when players know exactly where the enemy spawn points are or have memorized the boss's attack patterns/order, it's a solved encounter that has low repeatability.

• Choosing a scaling difficulty akin to Mistlock Instabilities or Queen's Gauntlet Gambits is also a great way to add replayability. This way, lower-skilled players are not left out and may play the content for a lesser reward or a lower chance of a reward. Meanwhile, those crazy Naked Bearbow solo Lupi runners can choose to amp up the difficulty.

• Rewards scaling to the degree of success, but not terminally punishing degrees of failure. Did you complete the encounter with no screw-ups (no objectives lost, within certain time, etc)? Good, max reward! However, there should be a way to optionally touch a mote and restart the encounter (not the whole dungeon/raid) if you think you can do better. Scaling rewards work great.

Also, a gimmick that I'd enjoy:

• Rewards that showcase how good you are. Anyone can complete a dungeon 9x for a legendary, buy an Aetherweapon from the tradepost, or get lucky in Fractals 17 and get that one Fractal weapon drop. However, games like Team Fortress 2 have the wonderful counters that show the amount of headshots, kills, etc with a weapon. GW2 can implement something similar with shoulderpads that change with kill markings on how many times you completed a raid, how many times you took Stonemist, etc. Maybe a weapon that gets different color ribbons attached depending on the most difficult or the highest number of "gambits" you completed the content with.

Overall, it'd be nice to reward the truly skilled players without disregarding the lesser-skilled yet tenacious players.