My attempt
Main menu:
Message of the Day spot no longer just used for Message of the Day. Instead, its an updating feed of info relative to me and my goal of world domination. Tell me if some bastard dares beat one of my scores. Tell me what my friends are playing. Obviously it can include Harmonix messages, but make it more.
Choose difficulty and instrument before I enter the quickplay menu.
Show me at all times if anybody on my friends list is playing. If they choose to in the options, let me just jump in to their game mid-song (assuming I have all the DLC in their playlist)
Hyperspeed setting saved. Different hyper speed settings for different instruments. Different hyperspeed speeds. All of this to be an option in-game and in the multiplayer lobbies.
Song select menu.
New option: sort by release date.
New option: Show songs that youve recently had your score beaten on.
Friend leaderboards displayed (obviously).
The option to challenge a friend from the menu., with a notification that appears in their main feed.
The option to select a friends score and have it displayed when you play, so you can see if youre close to beating it.
Voice recognition to search for songs.
Option to use a second controller with chatpad to type song name to search for.
By pressing start, I can change options. IE difficulty, instrument (on guitar), hyperspeed settings.
In multiplayer, simply dont show songs your internet partner doesnt own.
Party Play rip off.
In Game:
Different scroll speeds for local multiplayer
Option to have your highway be the same size as it is in single player, with the other players highway smaller.
Drum sticks counting you in out of pause menu
Option to calibrate from the pause menu
Song length indicator.
Display of your previous best score, and be kind of like a ghost mode, that counts up as you previously got points
Option to do that with friends score
Post song, option to view GH style detail on notes you hit/missed, whatever
Career mode:
Never make me play the same song more than once
Have songs in groups, but let me pick the order I play them (to a degree)
Jump in/out online play
Have challenges that are challenges, not just playlists
Silly options:
Let me disable songs
Let me disable transparency on the highways
Let me choose a venue and dont stop me from playing online because of it.