Currently, when doing synced content, the hardest is always the one from leveling dungeons. The reason is simple, the gap between "highest ilv at this level" and "average ilv people have for this encounter" is very small. Which achieves an interesting result, you cannot overgear leveling dungeons and you always have to interact with the content in a meaningful manner.

DPS/H can't stack in 3 AoE or be hit by a TB and survive
AoE deal more than 5% of the group's current hp
Boss last more than 25s.

I think it would be great for the game to have every piece of content, when done sync (through roulette for instance) to have the ilv being equal to the maximum ilv available from the content you're doing.

For instance, you're doing a dungeon whose gear drop is ilv 635. The maximum ilv is 635.
You're doing a story trial whose drop is ilv75. Your ilv is 75.
You're doing the current 24man raid or 8raid, you're synced to what the raid allows.

The goal isn't to really make things "harder", but to make the player interact with the content.
Basically, to make healer have something to heal, make tank feel like using CD on TB will have an actual impact (instead of "ohohoo, I lost 25% of my life on that TB that I didn't mitigate) and make the fight last long enough so that we can actually see something.

Players could still do the unsync with a full group if they wish to, that's fine. But the regular daily-weekly experience would I feel be more enjoyable of we'd actually need to "do" the fight, to engage.

Currently, trial and 8man raid feel more like glorified target dummy. You can litterally stand in half the attacks of the boss and survive. There's no dps check so you don't have to know how to play at all (fine for such easy encounter) and even if tanks forget their tank stance, a samurai can still tank just fine.