For The Undermine Journal, it would be cool to show lines from trade chat where items are mentioned, to see if people WTB or WTS things outside of the auction house. Trade chat is the public global chat channel for every realm anyway, so there are a lot of other neat chat stats one could pull from there.
I think an hourly pull, much like the current AH setup, would be great, but even a once daily pull would be useful. The sender's name and full message (including item links), along with a timestamp is all that would be necessary. It's the same stuff you could pull from Logs\WoWChatLog.txt, except instead of me having to have a character sitting logged in on every realm, I could instead just query the API.
I know this suggestion has a snowball's chance in Firelands, but it's worth a shot. Thanks.