If you have RB1/2 or GH:WT why would you ever buy another one?
For more songs? You can get new songs every week with DLC, plus you get to choose what they are.
For better/more instruments? People do not want two drum sets. What else can they reasonably add?
New feature? Nothing i can think of that people would want.
I think the weekly DLC devalues the newer games. If there were no DLC, people would be forced to buy the new version to get more songs.