Valindra is a fun choice for the one cover. She honestly shows up a lot, and is an antagonist that can be basically endlessly reused in near any situation.
Beyond has several smalls books that expand on things for sale, like the Mortuary released a little after Planescape.
So I expect them to be things like this
https://marketplace.dndbeyond.com/category/SRC-00125
The Gladiator is basically a mid-high level fighter. But one mid-high level fighter is not that massive a threat to multiple Players around level 5. The Archmage is the same being a near top level wizard, but that makes them a threat to mid level characters, not high level ones.
Here are the exact stated contents.
I also noticed we have what looks like a few new monsters, the Lizardfolk Warden (I am guessing a low level standard Lizardfolk), the Curate, Wight Lifedrinker, and maybe the Jelly Cube (if it's not just a renamed Gelatinous cube for some reason).