Great idea!
I also like a Shapeshifter/changer Powerset for Melee
Where you turn into different beasts depending on attack (and the different forms have different passives)
Examples:
Tiger Swipe
Turn into a Bipedal WereTiger and Swipe in a cone in front of you. Great Damage (cone) and does Bleed (DoT)-damage.
While in Tiger form all attacks do +15 % extra damage and have a chance to do a small dot.
Rhino Tackle
Turn into a Rhino that charges (either Targetted AOE och Location AOE (that you place)) to the Location/Target and Knocks back/down every target in the path.
While in Rhino-form you take 15 % less damage from Smashing/Lethal and your attacks have a chance to Knockback/down.
Gorilla Pound
Turn into a Gorilla , jump up a bit and Smash your Fists down into the Ground
PBaoE that knocks enemies down and cause them to become disoriented/stunned.
While in Gorilla-form all attacks have a chance to disorient enemies.
More animals can be added
And there are also Melee attacks that do not "shift" you, so you can stay in preferred form for as long as you like, but if you are in Gorilla and do Tiger Swipe, you change into Tiger-form.
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Another idea is to have the Animal-forms as Toggles a la Lethal, Fire, Poison-toggles on Dual Pistols, where the attacks themselves don't change you, but you gain different passives depending on form.