D&D 5E What would you put in Volume 2 of Tales of the Yawning Portal

[Parmandur]

Why? There aren't so many references (one in DMG and one in MM? Did I miss something?). Also although Great Modron March is a good adventure (not the best Planescape imho - i prefer Dead Gods) i think it is too niche for the 1/3rd of wizard's book production in a year.

There were mode than those two references, for sure: that and Modrons in general got a weird amount of attention on the books. A solid frame story to trapeze around the planes, at least...