Here’s some ideas concerning vaults.
When entering a vault to have a low chance to trigger “nightmare vault discovered” effect which makes monsters deal more damage and have more hp and at the end of the vault, after defeating the final boss to have a cage with a guaranteed T4 in it.
Some small parts in the vault, like certain rooms to have all the normal spawns replaced with treasure hunters (3skull 5 npcs that attack together) that drop eldarium, gold, valuable stuf in general but are significantly harder to defeat.
Enter a vault and randomly trigger “hardcore mode”. If you die, you get teleported outside (without your stuf) and the vault is locked for 10min (just like normal reset). You can opt out by simply exiting the vault. The rewards would be increased.
Have a small chance to find a hoarder insert vault race (hoarder goblin, hoarder wolfmen, hoarder lizard, etc) that drops 10 times the average loot.
Add more triggers that have to be shot with a bow. Shooting a bow is alot of fun and could see this working really well for unlocking secret chests inside vaults.
Randomly find a trader inside the vaults that spawns for a limited time and can give you a reward in exchange for eldarium and a free game tip.
Find scroll tables that can be unlocked with a fragment of power and can have you choose between 3 random feat scrolls (from the usual final chest pool table).
Find liquid jars inside vaults that when broken near monsters, would turn those monsters in demonic versions, just like we have with pets. These demonic monsters would drop demon blood, fragment of power and a chance at beastial memory potions.
Add thrall cages inside vaults with an increase chance to get a T4. But you have to provide the key from the outside. This is endgame lvl 60 content afterall and should have T4.
Some of these ideas come from the thought that exploration should be rewarded. Most players rush through vaults for the final chest and I feel that we need more reasons to wander off from the beaten path when doing a vault.
Feel free to add your own.