Safe Spawn Block

I believe it would be a good idea to add a safe spawn block. For realms, it could be a feature much appreciated. It could only be crafted in normal mode, considering mobs in easy are not as challenging. If placed on top of a block of coal, there would only be a 5 block radius of the safe zone. With a block of iron, the safe zone could be extended to 15 block radius. So on and so forth. If that could happen, realms could be more enjoyable, as well as other multiplayer worlds. If there could be a more expensive version, maybe requiring a nether star, it could change the spawn point. I believe that feature could be useful considering we have never been able to change our spawn point in survival without cheats. Achievements could be easier to get without cheats on.