One thing I have been thinking about for some time is that once you are a bit into the endgame and have a really powerful build, the only thing to do is really doing crucible runs and clearing pestilence outbreaks. When you are at this point in the game atleast I feel like the most fun part is fighting challenging bosses. This is where the problem arises. In the crucible you are always fighting the same boss, the echo knight, which gets old very quickly. Furthermore before fighting the boss you need to mow down 8 levels with enemies that are pretty much like ants at that point. It feels like the most fun part is the smallest part of the crucible.
My suggestion is to add an unlockable version of the crucible with maybe 5 floors and only bosses, maybe a final floor with two bosses fighting together. This could be unlocked with gloam seeds from the senechal perhaps.
Maybe rewards could be mostly items that allow for endgame build experimentation (fallen embers, plagued gear etc)
I feel like this could be a cool addition to the crucible especifically for those players who have really maxed out on their builds and are in need of more challenges!