Speaking of the planet selection, how is it going to be handled? JJ2+ allows for multiple exits, but there isn't really a way to track which levels you have already played. So, when the player is returned to the planet selector, will they just have the option of replaying planets or what? Also, how will you handle progression in this case?

All I can see working is you either have a new weapon obtained on each planet (and you need all of them to play the final one), or you have some sort of branching system.