As he title says... Mob Stacking!
So lag has and will always be an issue so I'm suggesting that we should have a plugin that allows mob stacking.
Here is an example: StackMob
We all know that entity movements contributes to lag and I think that this plugin should help ease that up.
Considering the fact that Skyblock allows players to own spawners, this is a good idea.
Yes I'm looking at you guys too in Towny with your passive mob farms. Don't worry about insta killing the whole stack of mobs. If you watch the video in the example provided, it only kills one mob per attempt.
Add in the previous dumb mobs plugin back then to completely prevent mobs from moving. They're really not a threat considering we run on Easy difficulty (Yes we are on Hard but the mob spawning nerfs make this Easy) lol.