My Zune HD got something like this where you got one button on the left side that shows track and all the functions(forward, next song, volume and so on)even if the device is locked. Very handy and works better than my old ipod touch 1G.
N900 will soon be my only MP3 player as i get rid off the Zune HD so hoping to see something like this at some point.

Most important thing would still be portrait mode for media player. I really don't care about portrait in the menus or in the browser(i already did hold my Omnia HD and 5800 mostly in landscape), but at least to me who is most of the time moving when i'm lisening music from MP3 player, having portrait there makes sense to me as i don't need to use two hands for something very simple like selecting new track or whatever.