It looks like Harmonix is once again taking song requests to be included within the Rock Band library for the newly announce and upcoming Rock Band 4 which is set to release later this year in 2015. From the link below you can go on their site and put in which songs you'd like to see available within the next title which is awesome. Simply put in the title of the song and the band that produces the music, let's see some Walk the Moon! Anyways, you can put in as many requests as you want and feel free for hilarity sakes to get my theme song in there "Cookies Be Crumblin'" by "MotorV8" which you can listen to below. Additionally the link to request songs stands out below as well for you, lets make Rock Band 4 have an amazing setlist and future DLC!
Rock Band Song Request