You want Guitar Hero to make money again?
1) Strip the current generation engine back to the simpler way it looked and felt in the heyday of Guitar Hero 2. The old circular 'notes' were much easier to see. Strip out all the 'Super-combo!' announcements and over flashy lights that got in front of the fretboard and distracted players.
2) Make it much more party friendly - the game should come with all tracks unlocked straight off for party mode. Even in single player people are going to dislike certain songs - make them skippable. Accept that the majority of your audience already knows how to play and wants to skip to their favourites. Allow people to set up tracklists to play ahead of time instead of booting players to the menu every five minutes. Set it up so folks can save these tracklists for any time they want to play. Once players are using tracklists, instead of loading a new stage between tracks the engine should precache the next song before the current one finishes - no stage change needed. For parties, allow players to join in, drop out or switch instruments between songs or even mid song. Put in an option to turn off 'failing' a song so everyone can relax and jam.
3) Beta test the life out of this sucker. It should be easy to get a demo out online on the consoles for the customers who have the instrument controllers from previous GH games. Get feedback and incorporate the most suggested changes. Maybe incentivise this with prizes for the testers whose suggestions prove most popular.
4) Put together an initial song lineup of popular songs from a wide range that should have something for everyone - a selection of recent stuff, 90's classics, undiscovered indie gems etc. Even music from games might sell if it can be licenced.
By now, you should have a lean game engine that has sufficient graphics without being distracting that is both easy to play for a high score or for fun at parties, With an enticing track list and seamless background loading that minimises waiting and time in menus. If you don't have this already, go through steps 1-4 until you do.
5) Release the game. Make sure it is backwards compatible with the old peripherals as usual. Now that you have a defininitive version out there, NEVER EVER release another Guitar Hero. As long as the game works and the band graphics (which players quickly stop looking at anyway) are sufficiently entertaining, there is no need to waste money on developing another version.
6) With your definitive version out there, the money is in expansion packs. On top of the usual individual songs made available to download, once a year release an expansion with about a dozen new hits/old faves for about ten bucks.This really is the key - GH has moved from cash cow to declining star - the best way to make money is to trot out new content (expansion packs) preferably download only to minimise production & distribution costs while keeping running costs to a minimum (don't pay to make a game that already exists!)