While I'd rather not see a splat book for a long time, I'm sure one's inevitable, I just hope that instead of a looney menagerie of new classes and races they expand on the current races and classes with more subraces and class options. Fighters, for example, could support a whole pack of subclasses (merchants, gladiators, knights, etc) to add options without worrying so much about unbalancing the game.
And backgrounds. Loads of backgrounds. Those are great, one of my favorite parts of the new edition.
While I'd like to see some campaign setting re-released, I'd like to see them in more table-friendly forms. For one, hexmaps are great for table use (I'm hoping the map in the Starter Set is a preview of things to come). And instead of huge DM-oriented campaign setting materials, I'd like to see a line of player-oriented releases. A player's guide to the Realms, etc, would be great at getting the whole group on the same page about the world they're traveling in.
Thirdly, and by no means least, I'd love to see a line of "big book of adventures", maybe one book for each of the four Tiers of Adventure, each book containing a variety of say 6-10 adventures. As a DM, this would be a great resource for a "design-your-own adventure path".