Core:
Magic Series Supplement
Well after Weapons of Legacy, I assume something more specific to Armor/Shield is in the works, then a book on Staves and Wands, Rods perhaps, one on Wondrous Items, and lets not forget rings.
Genre Series Supplement (Generic settings book?)
After Battlefield Adventures, I would like to see a low-magic genre supplement perhaps, though IMO this is where we might see something like a city adventure genre, religious crusade genre, mage autocracy genre, etc.
Environment Series Supplement
I bought Frostburn, while certainily a nice book, I realize that I will use too little material from it to be worth it, so I will probably skip buying any more of these.
Forgotten Realms:
Don't play Forgotten Realms.
Eberron:
Bought the CB, but until someone in the group decides to run an Eberron campaign I ain't buy any more Eberron books
Other:
Sorry no psionic books for me, not to my taste. For old campaign settings, most of the old settings don't appeal to me Dark Sun, Spelljammer, Planescape, Birthright, etc., except for Greyhawk. Greyhawk I would buy, I would love to see Greyhawk get the full book treatment, 320 page hardback book, full color map, every region revised and updated, regional feats, spells, PrCs, races, etc... $49.99 or even $59.99 would be worth it to me.