The new pet slime really is unique from the other pets because it is the only one with a passive skill.
Why not give all pets an active skill and a passive buff?
Here are my suggestions:
Passive bonuses if you equip the ff. pets:
Hound - + Gold Find %
Imp - + Item Drop %
Fairy - + Luck %
For slimes, I suggest an offensive skill since slimes passive bonus does boost your element damage.
The skill I suggest is Goo Ball.
Slimes will gain an energy counter above it. When you or your hireling kills enemies energy points will be added to your slime(s) energy counter. At a minimum of 30 energy points your slime will fire a goo ball to the closest enemy view on screen. 8 seconds cooldown.
Goo Ball Damage depending on the energy points used. +1% MH Damage per energy points which means if your slime releases a goo ball with 300 energy points, it will deal 300% MH damage to targetted enemy. Max of 500 energy points. Slime gains 10 energy points every second until it will fire the goo ball.
Energy points gained from enemies per kill based on rarity:
Normal - +5 points each.
Magic - +10 points each.
Rare - +25 points each.
Epic - +50 points each.
Legend - +100 points each.
Mythic - +250 points each.