Here is a great Idea I spotted in a Post from Sardaukar.
I want a whole bunch of hidden dungeons. And with hidden I mean that they don’t glow like objectives like in D3, they’re not mentioned anywhere in the game guide and you basically have to stumble upon them. Some you can find after getting a tome somehow, others by listening to an NPC, some you need to get a couple of artififacts together (like the staff of herding, but with a lot better dungeon), some I want only visible during the day/night (ingame) or even a certain moonphase. Get creatieve, but give us different things to puzzle out, to accomplish instead of an unending number of Greater Rifts.
I added to this with One of the best things I have seen here, wonderful idea.
Even to the point that this type of Dungeon will never be seen in the same place by other people, say when a character is created a code number is linked to it " RND Code " so that you cannot just search for a post " I just found a map that no-one else has and this is where you find it "
But also with weather being a new thing maybe even take advantage of that for one of the hidden dungeons maybe cannot be found unless you are near it after a rain storm or you would never have seen the odd shaped foot prints.
Maybe it could be that a monster only feeds during a set of conditions like Moon Phase and again if you are not there at that time you can never find the entrance.
Wild Goose chase types where it starts as a rumor as you are leveling and you find-out more and more as you progress, so power leveled chars would never get all the hints and you have to get them all to unlock the opening.
I think plenty of others can come up with ideas that are better.