Media attachments: The basic stuff. Labeling tweets that have links to photos or videos in them. Or sticking the links to the photos and videos in the metadata itself. Or tagging the content of the photos in the tweets.
Hashtags: Migrate them to the allotted space for metadata.
Reviews: Add one to five-star ratings to tweets.
Finance: Make it easier to follow stocks for spikes or sudden fluctuations with a special category of tweets. In fact, make a special type of price alert tweet for all sorts of goods and services like plane flights and oil.
Coupons: Make a special category of coupon tweets so you know when hamburgers are half off in the neighborhood. Location data would make this especially powerful.
Music: Mark all tweets that refer to songs. Create Twitter song charts that span different services like Blip.fm and Hype Machine. Generate playlists from Twitter. Granted, these ideas are already out there. But people might be hesitant to tweet their favorite songs for fear of spamming their followers. Tagging the tweets would allow people to opt-in to ‘music-related’ content from friends.
Location: Yes, there is already location metadata. But there are different ways of thinking about places outside of lat-long coordinates or place names from local listings. What about all tweets about parks? Or tweets about schools? Traffic tweets? Finally tagging all the tweets that are check-ins?
News: A news story’s dateline (when a location is written in all caps at the beginning of story) has been a key piece of metadata paired along with copy for more than a century in newspapers. Now it can be put into the metadata of a tweeted headline. This solves a big problem: media organizations often tweet the headlines of stories about topics like French politics or airplanes being grounded in Europe because of the Icelandic volcano eruption. But these tweets may not actually contain the words ‘FRANCE’ or ‘ICELAND’ or ‘EUROPE,’ so they won’t get picked up by a cursory search. One could also tag tweets that are about the same ongoing news story like ‘michael-jackson’ in the case of the pop singer’s death.