Stalker Mob

The stalker mob, or just the stalker, would be a hostile mob that would "Stalk" the player. What I mean by this is the mob would follow the player, but outside of say a 25 block radius or so. Once a player is idle for too long or is doing some other task which would make them vulnerable, the stalker would attack them. If you are in a 100 block radius of your spawn point or your bed, the stalker would stay outside of that radius. I understand how difficult this would probably be to code, but I think this would make Minecraft much more terrifying. Imagine exploring far away from home, and you catch a glimpse of the stalker on a nearby hill or by a nearby tree, that would truly be terrifying.