My idea is a new type of liquid, an ooze liquid. In it slime can move as if it is normal air, but anything else moves as if it were water, only at half speed. In addition, if the pool of ooze is at least 2x2x1, a small slime can randomly spawn in it, the time around twice as slow as the mob spawner spawn time. A 3x3x1 pool can spawn a medium sized slime at 4x slower than the mob spawner spawn rate, as long as there is air, or more slime above it, increasing the total area to 3x3x3, like this:
(teal = air or ooze)
(green = ooze)
Finally, a 5x5x2 area of ooze can spawn a large slime at 6x slower than the mob spawner spawn rate. Like medium slimes, large slimes need head room to spawn. They need an overhead of 5x5x3 in order to spawn, either air or more ooze will do, like this:
As for the behaviour of ooze, ooze lakes would spawn at about the same rate as lava lakes. Ooze is instantly washed away by water, turning any still ooze blocks into still water blocks. Lava acts as it does with water. Like lava, ooze cannot be harvested infinitely. Ooze flows at half the speed of lava. Ooze can be harvested with a bucket, making an ooze filled bucket. O0ze filled buckets can also be created in a crafting menu using 8 slime balls and an empty bucket, like this:
(turquoise wool being slime balls, and iron being a bucket)
(teal = air or ooze)
(green = ooze)
Finally, a 5x5x2 area of ooze can spawn a large slime at 6x slower than the mob spawner spawn rate. Like medium slimes, large slimes need head room to spawn. They need an overhead of 5x5x3 in order to spawn, either air or more ooze will do, like this:
As for the behaviour of ooze, ooze lakes would spawn at about the same rate as lava lakes. Ooze is instantly washed away by water, turning any still ooze blocks into still water blocks. Lava acts as it does with water. Like lava, ooze cannot be harvested infinitely. Ooze flows at half the speed of lava. Ooze can be harvested with a bucket, making an ooze filled bucket. O0ze filled buckets can also be created in a crafting menu using 8 slime balls and an empty bucket, like this:
(turquoise wool being slime balls, and iron being a bucket)