So here are my thoughts on the part/difficulty selector:
We should combine all the part selection and difficulty selection together into one menu, rather than having separate menus for each individual choice (which works fine for single player but is messy in multiplayer). If there are multiple players then there will be multiple menus side by side; each player's controls will control their menu, and it will only advance when everyone's finalised their part and difficulty. The normal key controls (up, down, left, right, enter) will control the menu of the left-most player who isn't finalised yet (so you can still do it one player at a time if you use the normal controls, e.g. if one of the players is Jurgen).
First each player's menu will show the available parts to choose from, assuming there is more than one (otherwise it's skipped, just like it is already):
<Player name>
Select part
[] Guitar
[] Bass
The [] symbols are the sliced icons I talked about earlier, showing an icon for the instrument as well as what difficulties are available.
Then after choosing a part we get to the difficulty menu, as you'd expect. If the other players haven't chosen a part yet then the first player will still move on to difficulty selection and the others will stay back at part selection; you don't have to wait for everyone to choose their part before choosing difficulties.
Here's the difficulty selector. It will, of course, be skipped if there's only one difficulty to choose from.
<Player name>
Select difficulty
[] Expert Guitar
[] Hard Guitar
[] Medium Guitar
[] would now be the relevant non-sliced icons - the one beside Expert Guitar will be a guitar icon with a red background, and so on. Pressing the relevant "back" button (e.g. red fret) would take you back to part selection.
Then you get to the finalised screen, assuming there are still other players who aren't finalised yet:
<Player name>
Ready
[] Expert Guitar
You can still go back to change your difficulty but once everyone is finalised then you can't go back.
If a player only has one part to choose from (e.g. their controller is the drum controller) and one difficulty to choose from then they'll skip straight to Finalised and he'll have to wait for everyone else to pick.
