1) Make MUCH more then just songs available in the shop. There should be optional backgrounds, unlockable mods, new menu song, new menu layout, different arrow graphics, be creative.
2) Make unlocking secret songs unique. You should get one song for clearing 25% of the original songs (non-secret, non-shop), one for 50%, one of 75%, another for 100% and a final for beating every original and unlockable song. There should also be songs unlocked for achieving a certain combo on any song, ei: as soon as you get a 500 combo a new song is unlocked. Also clearing every song in a section could unlock another. Using 1 mod could unlock one, while 2, 3, 4 and 5 mods could unlock more. Maybe bring back those actually 'secret songs' where a pixel by pixel wide link could bring up a password (unique in both password and placement per user). Once again, those are jsut some things on the top of my head, be creative, the credits thing makes NO SENSE. Although having a few for credits wouldn't hurt, just not all of them. Also, after clearing the song that resulted in unlocking the song, have a flashing text that says 'New song unlocked' or something like that.
3) Scoring. 'Nuff said. Don't base it on combo.
4) Personal high score list. Have an option somewhere on the menu that will bring up every song you have completed, along with you rank, score and combo. Secret songs would be at the end of the list, listed as '?????' until unlocked. Songs not played yet, but playable would have the name listed, but just have a 0 for score and '/' for rank.
Those are my biggest concerns, I could probably give 50 suggestions but I have to stop somewhere
