Right now, difficulty (Peaceful through hard) appears to be an program-wide setting in Singleplayer. This causes obvious problems when trying to maintain one world in Peaceful, another in Normal, and a third in Hard -- I have to change the setting every time I switch worlds.
My suggestion is simply that this option be saved with gameworlds, and settable separately for each world.
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I did some CraftTweaker scripts for Mystical Agriculture. They fill in a couple of small gaps in MA, and also let you make or duplicate not only vanilla plants, but the blocks, plants and wood from Quark and Biomes O'Plenty. Also spawn eggs for most vanilla mobs! The scripts are here on Github.
This would really only be an "eh" feature, so then we have to go into the question of is it worth the time to program it. I personally just keep my difficulty on Hard at all times, and when I don't want mobs I just do /gamerule doMobSpawning false.
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"We live only to discover beauty. All else is a form of waiting." - Kahlil Gibran
My suggestion is simply that this option be saved with gameworlds, and settable separately for each world.
Is it really that hard to just go to your menu and just change you difficulty, please don't be so lazy
Click them or they will click you