How do you feel about vore/absorption fetishes? will you be adding anything like it?
A simple way to add this is slimes getting extra HP drain or stat boosts/ability unlocks from attacking/sexing up/defeating other slimes.
A more complex way would be for characters to fuse directly, resulting in a new (or temporary) character.
Or as a field summon... the green slimes could summon a motherslime boss onto the field by defeating or sexing up a slime girl.