I was thinking...
"Great to be Different - Forest Rain" and "Between Fairytales and Happy Endings - PrinceWhateverer" both gotta be done and added onto RBN... Really. They are both great emotional songs and are highly recognized by the Brony fanbase. Both has no legal issues and should be accepted by Harmonix.
I also still believe "Rainbow Factory - Woodentoaster" and "Awoken - H8_Seed + Woodentoaster" gotta be added onto RBN but shouldn't be like primary ones to do, even though "Awoken" would be bucking amazing to have and be accepted by Harmonix easily. "Rainbow Factory" might be a problem though since the song does have the word 'Cloudsdale'.
Also, "I Am No Hero - Mandopony" can be added onto RBN without problems as well.
"Spirit of Chaos - Aviators" should be accepted easily as well and is a good song to do. The song's opposite "One Last Letter - Aviators" might have a problem though cause it does say 'Discord'. I can't remember if it mentions Celestia or not... Actually, 'Discord' should be accepted since we kinda have a Brony song already that says 'Discord' a lot.
"Everything's Magic - PrinceWhateverer + ShadyVox" would be a nice and accepted addition as well. "Shining Armor - Mandopony" can also be accepted because technically, it is saying shining armor as in a knightly protector, not Shining Armor, the pony. But Harmonix can still be quite picky...
Just naming a lot of songs that I personally think would be great additions and any type of problems they might have with Harmonix and legal issues.