Some good ideas already, but I'll throw in a few too.
ARMS LAW - I agree with Vlad to some extent here. I'd like to see Weapon and Armour specific crit tables which dovetail into a piecemeal armour system. So you'd end up with a situation where the general level of armour coverage and the body specific coverage come into play when considering attacks and criticals. I have no problem with separate attack and critical tables. A lot of work, but it would be awesome.
SKILL LAW - There have been a few requests for more detail regarding things like Healing, Herbs, Subterfuge, Social (trading, diplomacy etc), Outdoor, plus Firefighting and other more obscure skills. I think that many of these could be included in a book of that nature. SOHK was a great idea, but it mostly regurgitated old material and barely scratched the surface when it comes to the potential details of these skills. As far as possible, I would organise it by related skills rather than alphabetically.
MONSTER COMPENDIUM - More beasties, and more scope for using monsters as characters (NPC or otherwise).
SETTING MATERIAL - I tend to devise my own scenarios and campaigns, but I think a book of short adventures (short modules that could be incorporated into a campaign...it could go from the 'obligatory' 1st level rodent bashing mission for the publican, right up to short diversions for the high level character), NPCs (a detailed NPC chart would be a good start), and world building/scenario ideas. These would interest me as both a consumer and contributor.
Apart from the core rule book of any new system (which I'll discuss in the appropriate threads), I would like to see all material include a GM section which details ways that new material can be added and yet still fit into the system (eg new weapons, races, skills etc). I would then encourage players to share their work via the website (I was amazed to see how much great material - better than the original game - the aurora toolset for Neverwinter Nights produced). Having an established method for home brew material could keep players in the fold, and provide a dynamic exchange of ideas.
In the case of most of these suggestions, I have a home brew system I already use (Piecemeal Armour, Injury and Healing, NPC table, and of course setting material). I would be happy to contribute my ideas, but would be even happier to see what the professionals come up with.