I'd like to see a GUI based Speedtest under the Tools section of the firmware. This way if i'm offsite I can remote in to the router and get a visual speedtest directly off the router.
Even if you make it just a text based one as long as it was built in the the Web Interface under the Tools section it would be nice.
If you didn't want to make it use an existing speedtest site you could always make it configurable. For example the user would be required to put in their own FTP server info and the speedtest would just upload files of certain sizes to the FTP server and then download them back while calculating the time it took. Then it could spit out the information and viola! Easy speedtest. Results would still be affected by FTP server location and utilization on that server etc… But a lot of speedtest sites have the same potential issues.