If we're talking about pure money-making, then it's time for 4th Edition. Nothing else in the RPG line will generate as much money as quickly. If, however, they feel it's too soon for 4th Edition, then they would do well to do another range of 'Complete' books and 'Races of' books. Monster Manual V (or Fiend Folio II) is also a near certainty. And a PHBIII and/or DMGIII seem likely candidates.
Plus, of course, there's always the minis.
If, however, we were talking about what I think Wizards 'should' produce in the next year... well, I've actually been quite pleased with their output in the past year, so would like more of the same, please. In particular, I would like to see more adventures (and more new adventures - I know nostalgia is a powerful force, but I'm getting a bit burned out on it). Keep producing them, and keep the quality up, and I'll continue buying.
Some particular products I would like to have, that I think might actually have a chance of being made:
High-level Campaigns: a how-to guide on expanding your campaign into high levels (13 - 20), with especial focus on making the DM's job easier, flowcharts for quickly and effectively advancing monsters and NPCs, analyses of different types of monsters and their roles in the game, advice on managing the complexity of dozens of sourcebooks, etc. Some or all of this may have already been covered in "Power of Faerun".
An environment book for Swamps and/or Jungles.
A 'monster' book on Giants.
Another monster book on Orcs, Goblins, and other 'classic' humanoids.
A Feat compendium. Possibly a Prestige Class compendium too.
And some 'dream products' that I would very much like to have, but I don't think will ever exist:
Heroes of the Ages: A guide to running multi-generational campaigns, following the exploits not just of a PC, but an entire adventuring dynasty across centuries of game-time.
A very good, very easy to use PC application for quickly and easily advancing monsters and NPCs. Must include all data from all WotC books to be useful, and must allow the DM to implement his house rules in a meaningful manner. (As I said, products I don't think will ever exist.)
Dungeons & Dragons, Encyclopedia edition: Take the core 3.5 rules and put them in a volume. Likewise, gather all the PC races, base classes, prestige classes, and so on. The monsters would take multiple volumes, but once done the entire game would be present in about 25 volumes, all fully gathered and cross-referenced. Of course, the major downside of this product is that it would be the very last thing I would ever buy for 3rd Edition. (Heck, even if it's just a big subset of the rules, gathered to offer the fullest 'classic' version of the game, I would buy... but it would need to include psionics, although Incarnum, Epics, Tome of Magic/Battle and Weapons of Legacy could be omitted in my copy

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