My theory-
I was reading thru battle.net, and i noticed that they said something along the lines of-
This new challenge spans thru Diablo's bosses, from the Den of evil to Baals throne room.
I was thinking, what if when you had the SOH in your inventory, could differen't keys drop from den of evil? mabey travalence? why not baal's minions. It could be the next step, you would have to have completed the 1st steps up to Uber Trist which you get standard from, you keep standard in your inventory and bam you can get more keys...