I think the Crucible is a great addition to the game. It’s a great way to hunt for items, stock up on rare crafting materials and it presents a higher challenge than the Campaign by pitting you against monster compositions you would never face otherwise.
That said, it get’s pretty monotonous pretty fast since it’s always the exact same stuff per wave.
So how about this:
When talking to the Arena Master, why not present an option for Randomized Spawns?
The way I imagine this, is that the general composition on each wave stays the same. For example Wave 100 still has 1 - X nemesis spawns based on difficulty, wave 149 still has 5 bosses and so on. But simply randomize the spawns. Instead of always having the same pool of, I believe, 10 specific bosses on Wave 149, just have those 5 bosses be any 5 out of all bosses available in the crucible.
Same thing for the regular and champion spawns. Just hit /random and spawn whatever you rolled.
This may make waves a lot easier or a lot harder, both may be equally probable outcomes. So maybe there is no need to adjust the rewards at the end of the run. It would just be for fun, for some extra challenge and to make each run less predictable.
Plus: instead of people getting slammed by 3 Iron Maidens on Wave 150, they may already get slammed by 2 - 5 Mad Queens on Wave 149 (cue evil laughter!) 
Edit: Ok, MAYBE put a cap on how many of the same boss can spawn on each given wave. MAYBE…