Alright, based on the few games I've played with 3B, some additions to the game that would probably add a lot that I haven't immediately seen scanning the ideas thread.
1) Right now it appears to me that trade consists of a steady stream of income. At least I haven't seen one time boosts every time a caravan comes into my city, so I assume that is how it works. Trade should be a one time boost on each end. This would be more realistic, and allow you as a hostile player to choke trade out at either end more easily, instead of having to hunt down those individual caravans.
2) Tech diffusion via trade. Since tech is so dependent on the sheer volume of territory one occupies now (with the corresponding libraries) a small empire risks by an empire being more numerous and more advanced (with one advantage, why the other?). A good solution for this is have caravans running back and forth between empires carry tech points. If you are a technologically inferior empire trading with a superior one, every time a caravan comes into your city from that more advanced empire you get tech points, scaled on the difference in overall tech levels between your two empires. In return, they could get a slight boost in cash (signifying the greater value of their more technologically advanced goods).
3) I don't think there is enough to do with characters that is specific to them. Adventurers, I got it. But merchant NPC's I should be able to send and form a fast or high performing caravan to another nation. They shouldn't just be some random dude who happens to earn two gildars a turn randomly. Researchers should be able to do something special (create items or something). Loremasters should be able to do magical constructions, or something of that nature. These characters should feel different based on their job, and right now they don't. I think there is room for a lot more depth here, and as I have stated on other threads this part of the game really needs to be expanded, if not prior to release (as we are running out of time) then subsequently.
4) I really like the new economy. That being said, I think there is room for improvement. Right now your characters and NPC's don't play into the economy at all. They should. NPC's who are not under your control should greatly outnumber the ones who are, and they should be doing stuff (adventuring, trading, getting mercenary contracts), and using the money made from that to buy stuff in your cities, recruit your citizens to help them, ect. I've stressed many times how including NPC's into your living world is important, and I stand by that. Once again, something to think about for the expansion.
Anyways, that's all I've got right now. I love the new economy, and I think that spell research is very much heading in the right direction as well. We are a little late to add the last suggestion in, but the other three are very doable (for number 3 getting the AI to work it would probably be difficult, I don't know), but I think all of these things would add quite a bit to the game.