I thought this is a cool idea, that would solve a few issues.
Maybe there could be random times, like every 10 days or so, when the mobs will actively, in great numbers, and with new behaviors, seek out the player, and attempt to bypass their compound/structures to get to the player.
It would happen spontaneously; the player would hear sounds of frenzied mobs, and they would spawn by the multitudes. Maybe some would have pick-axes and mine to the player, and/or creepers could blast in for the other mobs; mobs could also errect ladders to a certain height, and open wooden doors to find the player.
It would add a point to all the security features people have in their bases. In the game as is, once you torch up an area brightly, none of your security in your base, like airlocks or fences or traps or blast walls really matter. All of your traps and security features are just novelties. But if mobs actually actively put the player under seige, players could actually put these to use. You could build a panic-room or a fortress, which you would run to whenever the mobs frenzied up, and battle or wait them out. You would have to have security soluons in your mines too, incase you got trapped down there. I think it would add alot to the game.
Didn't Notch say that during a zombie siege the zombie's could break blocks? I'm not trying to kill your idea sunflower but it's only the different behaviour that would set your idea apart from Zombie Siege
Okay so then mabey Notch can add the new behaivors to the Zombie Siege idea beacuse this is an awesome (but hard) idea sunflower has