i have some mobs which might be good
sponge zombie? drops sponge pieces which can be crafted into sponge
sponge skele? drops sponge arrows/bones arrows do 40% less damage but bounce 2 times
spongerman! drops sponge orbs which when merged with ender pearls create ender sponge which could create tools/armour maybe?
a few mob ideas :)

Not a bad idea. if you add armor/items, maybe add Sponge boots that decrease fall damage. Also, maybe make a new kind of spongeblock that is either bouncy or softens falls, or change the properties of the current sponge block to make it do just that.

YES! A biome made outta useless! :D

Maybe add an item that you get a chance to get from breaking a sponge block (1/25). It's called "Mysterious Essence" and can be crafted with any ingot or gem to make items made with them more enchantable (When used on a gold sword it can give you a chance to get Sharpness IV/V with only 15 levels)

add trees called "Dead Tree" and allow the wood that youg et out of the tree to only cook charcoal.

Reasoning: It's probably called "Dead tree" because in a sponge biome, it is very unlikely that the tree will get any water, causing the tree to die and its wood to be dead also. the Dead Wood should only make charcoal because it is dry and hard. you will not be able to make any sturdy objects with that wood. it should be very flammable .