It would be great if Smartlists could update relative to the current master (or most recently played) track. So, for example, we could create a playlist that includes all songs where the tempo is within 10% of the currently playing track's tempo. Or a list that includes all tracks with adjacent keys to the current track. Or both! Maybe this would be a more flexible option than something like the Match feature in Traktor DJ 2 (which I love, personally) that might make it appeal to more users.