Having had a look around the forums, I found a few threads devoted to improving the Diplomacy Tree; I thought I would go ahead and try to create a meta-post for those interested in the topic. NOTE: I have tried to keep these restricted to post-release stuff focused on concrete changes. If you think yours is missing, just reply with a link.
Other Possible Diplomatic Techs:
This thread has an interesting discussion of other possible techs, including a variant of my suggestion for techs to unlock further units from Ally resources.
IMHO the best ideas from that discussion are:
1) To create a propaganda branch that allows you to increase your prestige / moral, or to lower the prestige / moral of other factions.
2) To add an espionage branch that allows for creating spy units, stealing resources, etc.
3) To give a building that would enable election style victory. Maybe a Council of Magi, that has the power to select a Supreme Channeler?
Link to a thought about what a spy unit might look like:
I also like the idea of having a tech to enable map trading, hat tip to:
And potentially having another type of treaty for Open Borders. The lack of something like this feels like a major oversight to me, as it closes off large areas of the map if you are surrounded by allies.
Diplomacy and the AI:
Here are two posts that I think nicely lay out some of the problems the AI is currently having with Diplomacy.
Interesting Diplomacy Concepts
I think the idea of competitive auctions between kingdoms could be very interesting, and add lots of flavor "Come on come all and bid of the Lich Kings Crown! I can't wear it so I am going to sell it!" Gaining access to such auctions could potentially be a Diplomacy tech, maybe associated with an Auction House improvement you would build in a city.
I like the concept of Diplomatic pressure, although balancing implementation seems hard at first glance.
Other ideas on how to improve Diplomacy?