Rock Band 3 will be bringing keyboards and Pro Mode to the table when it's released on October 26 but what about all of the songs that don't support the new features? Harmonix seems to have an idea about what to do with them and wants the fans to chime in with what tracks should be updated.
While you will be able to play "legacy" songs in Rock Band 3, they won't have dedicated note charts for keyboards or any instrument in Pro Mode (however, every DLC song released after Rock Band 2 already has Pro charts for drums so those can be played on day one if you've got the cymbal expansions). What Harmonix wants to know is which songs are most deserving to be updated first so they can make use of all the new features as soon as possible.
To make your suggestion, you can post at the Rock Band forums or try your luck with the song request form. As long as you're polite about it, you might as well use both. And you vocalists out there shouldn't forget these updates can bring in three-part harmonies so be sure to make yourselves heard.