Here's my predictions based solely off what I hear and read (no real "insider" information):
D&D will continue to sell well with new products like DMG2, Weapons of Legacy, and so on. I expect to see another BIG SURPRISE from WotC early in the summer about a major product coming out in the fall. I don't know if it will be a new campaign setting or not, but it will not be a new edition (or even a hint thereof).
Eberron and FR will continue to sell equally well. Eberron may see a slight increase in sales after D&D Online comes out, but chances are that the MMORPG will not affect the sales of the print line whatsoever. I do think the possibility is there for D&D Online to be the big release in online gaming next year.
The minis line will continue to take the collectible minis market over. Each new set will continue to feature better sculpts and painting in a field they are already beginning to surpass the big dog (MageKnight and all the other "Clix" games). I would actually expect to see some premium figures out sometime as well. By premium I mean, larger, non-random figs with a higher production value, but still put out with stats cards and be ready to play. Then again, I don't expect it, but I hope to see that someday.
D20 Modern/Future/Past/Apocalypse will actually see some supplements. Not many more than 2 books, and they may actually be softcovers, but Modern fans won't have to rely on other publishers alone for support.
D20 in general will not see nearly as many releases in leiu of bigger, higher quality books. There will be even more chatter that D20 is dying, but in truth what is released will be better and draw more people to the systems. I think we'll see more factioning of the D20 market with OGL products like Grim Tales, M&M, Blue Rose, and others seeing more support than D&D. There will still be generic D&D support, but releases will be fewer and farther between. Mongoose will come out with a couple more lisenced products, but sadly their editing won't be any better than normal.
Warhammer FRP will see a lot of buzz and be a great product, but I think it will die out rather quickly. The setting hasn't ever been able to hold the appeal of gamer beyond the wargamer crowd. I wouldn't expect to see it draw many wargamers into the RPG camp, but there will be a few. (Then there will be another upswing of posts about roll-playing vs role-playing.

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Castles and Crusades will sell well, but it won't be the big hit of the year. It will sell very well and generate a lot of buzz, until groups hit the little things that nearly every "rules lite" system doesn't take into account. (Those little things are different for every system.) If C&C can address these issues, then it will do well, but if not it'll see only a brief moment in the sun.
WoD will sell well no matter what. Exalted will continue to sell well also, but we'll see a slowing of support until a new edition comes out in '06 (by my guess).
Palladium will survive, but will only see 2, maybe 3, releases in the entire year. Chaosium will pretty much do the same. However, I do think that sales will only be good among the diehards that stick by these companies even though they aren't ran very well at all.
Overall, I think that it will be a good year to into gaming no matter what game you play.
I lay no money on these predictions, it's just my best guess.
Kane