Here's the official rundown of what's finalized enough to be announced:
Mech Manual: You know about this one.
Second Age of Walkers: Scheduled for March '05. This is a 192-page campaign guide for the DragonMech world. It's divided into four main sections, focusing on the Stenian Confederacy, Legion, Irontooth Clans, and L'arile Nation. Each section covers the history, culture, philosophy, geography, military, and other details of that faction, as well as crunchy bits necessary to play that faction. There are new feats and mechs for characters from each section; other new rules as appropriate (such as spells for the L'arile and Legion sections); and a host of NPCs, particularly for the Irontooth. And, of course, there are the city-mechs! Second Age includes a detailed look at Durgan-lok, Tannanliel, and Haven.
Steam Warriors: Scheduled for June '05. This 128-page sourcebook expands the options for the steam-driven classes (coglayers, mech jockeys, and steamborgs), as well as general steam-driven options in the DragonMech world. It covers a wide variety of topics ranging from new feats, new steam powers, new uses for skills, new equipment, new prestige classes, and so on, and even has a section on playing cogling characters.
Other books further on the horizon will look into the city of Edge in greater detail, and explore more "monstrous mech" opponents.
I like the idea of a deluxe city-mech book. There's a good mech cross-section illustration at the end of the Mech Manual but I still haven't gotten a chance to really do a super-cool detailed mapping of a city-mech. That's a good idea.
As for adventure modules, no more in the works, though I have been thinking about digging out some of the old home-use adventures I wrote for DragonMech early on. I'll see if I can find any...