by slemon » Thu May 14, 2015 23:15
Do a time-travelling mod. First, it should record all entities' movements and block changes. Then, if a chat command is executed, that mod shall do a global rollback and create entities that were at a time player wants to travel to (mod should create entity for the player too). After that it should unrollback world in real time.
If that's too hard, I would suggest doing Robot Servent Mod.