For those unfamiliar with the Rock Band Network, it was basically a platform which allowed content creators to import their own tracks into the game and sell it to other gamers. It allowed Harmonix to create an ecosystem that would constantly grow with new songs without them having to spend too much developmental resources on it.
However it should be noted that Rock Band Network will not be returning in full, but rather only select songs will be returning. Also unlike the Rock Band DLC, gamers will not be able to carry over previous purchases to the newer game due to royalty agreements. This will also only be applicable to the PS4 and Xbox One version of the game.
There is some good news and that is Harmonix is taking requests for songs to be featured in Rock Band. There’s no guarantee that all requests will be entertained, but if you’re interested you can submit a request via Harmonix’s website.