What I'd like most to see are the Historica Geographica Series greatly expanded, with at least one coming out a year. I know we're getting the Blessed Lands and the rereleased Ssethregore sometime between now and September, and there is the Kio-mini Sourcebook coming out in a vaguely similar timeline, but I just want more (to quote the internet PUT IT IN MY EYES!!!!). As much as the epic volume of the Codex Arcanis 2 (the Revengening?) interests me, I would greatly prefer to see each section get its own book similar to how Canceri and Ssethregore did. I'd love to see a Khitani Book, a Coryani Book, a Milandir/Almeric book, a new Legacy of Damnation/Unsealed Lands Book, a Western Lands/Uggur/whatever else book, etc. These more 'personal' documents give more room to explore the cultures and histories of these nations rather than having to crowbar them into a much more (relatively) limited page count.
Also, as stated before, I'm FAR more willing to buy 5x $15 books over a year than a single $50 book, if only because I'm stupid and can't do math
It is harder to rationalize single big-money spendings, but much easier to rationalize smaller ones spread out over a year, even if the final monies are the same.
Another thing I'd LOVE to see would be a new Psionics Unbound-type book, and (as a player of a Divine caster) that Codex of Faith idea which was dropped a while ago, but hasn't made it (officially) past the title and general idea stage from what I can tell. We're already getting an Elder Sorcery book from the Kickstarter, so these types of books seem completely possible to me.