I've done a quick review of the WordPress Schema. I'm a user of WordPress for my own blog, and a Database Modeller by profession.
I'd like to help in some way to improve WordPress, however my comments, suggestions and ideas only go part way to being able to be immediately implemented due to Potential Code changes, and testing would be required, however I've stated my ideas in stages.
For conviencence, I've documented some initial comments which you can read at Improving Open Source Databases - WordPress.
Please contact me if I can assist in the implementation of any ideas, or perform a further more indepth review.