Remathilis said:
Eberron is bound to get another book beyond RoE. I'm thinking the next book will be Xen'drik or the Mournlands. These are two elements highly discussed in Eberron products. I also think you'll see a Dragonmarked Book soon.
I think all those are likely. Xen'drik is the setting for the D&D MMORPG (sort of a strange choice as it's not really a good showcase for mainstream D&D, IMO, but whatever), so I'd expect a book for that to come out close to open beta (if they have one) of that game, playing off the info the developers were given. I also wouldn't be surprised to see a book covering Sarlona and Argonnessen, but likewise, I wouldn't be surprised to see them sit on those two continents for a long, long, long time. The Eberron planes are interesting enough to merit a book as well -- or maybe just some modules set out there.
The Monster-Book Series will probably go with Giants, Fey, or Planar Monsters (Fiends mayhaps?) I really can't see animals, monsterous beasts, or plants getting there own title.
I'd love Giants and Fey, but I'd really, really, really like to see elementals and genies get a book. Bring back Janti as an official language, bring back the Shi'ar, update the Elemental Savant (and make it less suck), give the Bonded Summoner more coverage, bring the tasked genies from Al'Qadim back to 3.x, etc.
The Enviromental Series will see another two books: An Aquatic Setting and an Jungle Setting. Mark my words.
I sure hope so. The "Far Corners of the World" series on the WotC site seems to be running out of steam a little bit at this point, IMO, although I've enjoyed it all along.
There might be another Monster Manual, but don't expect it to have a number convention. What hasn't been used at this point?
Er, IV to infinity?
I like that MM3 essentially works as an Eberron monster book. I hope they continue that trend in future monster books. It may be just me, but I also wouldn't mind if they went back and brought MM2 and FF into 3.5 compliance. People could keep their old ones and the downloaded errata if they wanted, but some of us would like to have the proper versions between two hard covers.