I think it's a pretty safe bet that Harmonix has created new gameplay for Beatles:Rock Band. They wouldn't just "skin" the existing Rock Band engine with character models, instruments and songs and call it a day.
So, what does this mean for a game about the Beatles? Consider:
1. All singers also played instruments
2. Most songs feature vocal harmonies, especially the early '60s stuff like I Want To Hold Your Hand, Love Me Do etc.
3. Rhythm guitar is featured prominently
We could potentially see
1. "Voxtar" as a mode, with both vocal and guitar/bass charts assigned to a single player
2. Multiple vocal parts
3. Rhythm guitar making a comeback from Guitar Hero 1 & 2
This adds up to songs potentially having 5-7 parts. That's pretty complex stuff, but you could give people an entry point by letting them play in the usual vox/gtr/bass/drums configuration with the additional parts as a backing track. Then, offer all charts as an optional, maximum-scoring "Expert-Plus" mode.
Voxtar and harmonizing aren't just "cool and different features," they make playing Beatles: Rock Band more like being a member of Beatles: The Real Band, which is a pretty good One Question for this game if you ask me.
Hey, I can dream.