If we were just going by names, I'd say "Marathon" would be the Modron March, but as dice are being released alongside "Broadway", that's a decent guess.
One further thing though that I just though of - the artist on Reddit said that of the two books they knew about, one was full of monsters and the other was based in the Forgotten Realms. Now, I may be parsing things a bit, that seems to indicate that the one full of monsters is not based in the Realms, which leads me to think either: (a) a Volo's type book focusing on creatures of the planes, or (b) a "settings" book, with sections on various settings containing a bestiary of setting-specific creatures. Given that we've had a lot of hints at upcoming planar stuff, I wouldn't be surprised if what we're getting is the former. If so, we'll likely get a book that details the various ecologies of devils, demons, celestials, modrons, and so on, a section for PC rules (planar races, perhaps some planar backgrounds), and finally a bestiary section with various extraplanar monsters which haven't been updated to 5e yet - various remaining demons and devils, hierarch modrons, all the yugoloths that were left out of the MM, demodands/gehreleths, and all the various missing celestials (archons, guardinals, and eladrin), as well as miscellaneous still-un-updated planar creatures (xag-ya and xeg-yi perhaps?). Maybe even a section on the fey as well...
