Now after 1.4 I am sure many know that Slimes spawn in swamps, but cause in the main world all mobs are disabled I thought why not enable those in the Mine World, another little thing that only the Mine World can give you.

And when it comes to Villagers idk why they are disabled, villager trading were fixed according to most moderators so why are they still disabled, I want emeralds to be less rare and cost less, but without villagers that is not really possible.

I'm not so confident about it,but I think there's a mcmmo rank where you can get slime balls,but I'm not sure.
I have to say I like this idea,because there are barely any mobs in the mining word.

or ask admins to create a mob world.
, which contains mobs for u to kill/grind.
that would me good for me. 

The only purpose (besides experience points) of slimes if for the slimeballs they drop, which are used to make sticky pistons (and maybe potions, but i'm not a potion's guru). If pistons are enabled, I would like slimes added as well.

adding a whole new world is a bit obsure since worlds cause lag and for every new one we get = more lag so just having them in mining would be good enough