Imagine there is a hidden dungeon in the game. You can acces it only while doing vet random dungeon and there is only a small chance that you will be possible to get there. Lets say the final boss in the dungeon can spawn a portal after death (I think that would be the easiest to implement , the chance could be higher in dlc dungeons, but still more like 2-5%). You can enter the dungeon only with your current group. Hidden dungeon is more like a DLC dungeon and has some cool mechanics. Its not long but has few bosses fights. Lets add even more to that. For example you cannot wipe in the dungeon because it will throw you out (necro tank could be useful

. The final boss drops a cool reward. For example an "always meta" monster set/rare furniture etc.
The idea is about a rare opportunity to acces to something and have a chance for its rewards. I realise that sort of things where already in other games, but why not?
The way I described it is the way I would like it to be so it's only my personal approach to the main concept. I like what they did in last DLC dungeons with "hidden" stuff, so how about going one step further?