I liked the Burn a lot as a dungeon, but I didn't consider it at all difficult. Haven't done Arboretum yet (I plan on it tonight), but I expect it to be similar. None of the dungeons in the Expert Roulette since early in ARR's lifespan have felt like they deserved the name. I expect that'll sadly remain the case, unless SE begins adding optional objectives or imposes severe iLevel caps.

I think the best we can hope for is, in my mind, exactly what The Burn offered: a beautiful map, a refreshing sort of feel, and mechanics that walked the line of requiring attention to be paid, but not feeling overly stressful.