I hate to the the guy to ask for more once we're given something good, but as someone who's cleared criterion+criterion savage, I'd like a similar challenge with solo players. My main issue with PotD and HoH solo as "hard content" is that the difficulty is largely from sunk cost, where a single run can cost you multiple hours if you wipe, which is no fun to me.

A solo variant (or wipeless criterion) run can be 15-25 minutes though, which is a huge benefit when it comes to small-form hard content. It would be cool to see even lower-party-member form content though like a solo criterion dungeon (which gave maybe 1 variant action to equalize dps/survivability among roles), so that there was a way to run hard content solo that could be attempted/learned in 20 minute bursts, without losing hours if you failed.

Even without that though, Criterion+Savage are great, I just wish the rewards were maybe a little better / gave more tomes or something.