Get notified when mobs die! The message would be something like: Player killed Creeper with a Diamond Sword in World. Player killed Ghast with a Bow in World_Nether. etc... or even... Creeper blew up on a skeleton. Skeleton killed Creeper. etc...
That would literally only take about 15-30 minutes to make, if no-one else wants to, I might make it.
Should be easy but this could go crazy and lag the server if anyone has a mob trap/grinder. Also imagine it had just been night then the sun came out, imagine how many messages would be sent, especially on a big server.