I doubt it would be unlocked as you progress through Single Player.  From what I've heard, SP is essentially a tournament a la fighting games.  The only real difference between SP games and MP games is that everyone is an AI and you get a neat cutscene when you beat it with a specific Demigod.

However, I do believe that these would be cool additions to the game.  Perhaps instead of SP milestones they could be tied to Favor.  As has been addressed several times, you can get all the Favor Items with a fairly low amount of Favor.  The only reason most beta testers don't have everything unlocked is because the system is bugged and steals Favor, de-unlocks items, and doesn't always add Favor after a game.

New soundtracks or graphical elements would be neat ways to spend your Favor Points.

I think the title screen has a very simplistic appeal. Once they added the subtle texture to the angels and made the sky move, I felt it was great. But being so simple, it seems easy to impliment a gradient of colors, from the impulse clock, going from dawn to mid-day, sundown to dusk, evening to midnight. It would be VERY cool.