I have been a fan of pokemon since red and blue first came out, so I'm gonna give you my input on how I think darkrai should work.

Basic info:
Darkrai is the same weight as lucario, but stat-wise, Darkrai has very high speed and special attack, and the rest of his stats are about average. He is able to fly very quickly, and has stilt-like legs that extend so he can stand on the ground. He has low attack, so physical attacks do minimal damage.

Attacks:
-Forward B:Dream Eater: Speeds forwards and passes through players (he is hittable in this form, and if he gets hit, the attack is canceled). anyone sleeping in his path is woken, damaged slightly (10-20%), and Darkrai is healed by half the dealed damage.

-Neutral B:Dark Void: Charges a sphere of dark energy that  is thrown, envelops the enemy and quickly disappears, putting them to sleep. (see movie for details)

-Down-B: Sinks into the ground and becomes immune to attacks that do not strike the ground. he is seen only as his shadow on the ground.

-Up-B: Similar to Pit. Allows Darkrai to freely move about in the air temporarily.

-Smash Up-A: Releases a small burst of dark energy upwards.

-Smash Down-A: Dark Pulse: charges a dark pulse, then fires it in both directions.

-Smash Forward-A: Shadow Claw: Slashes the enemy with a claw of shadow energy.

Ability:
All sleeping enemies are dealt small amounts of damage constantly.

Fiinal Smash: In my opinion, his smash attack should be him flying to the top of the screen, charging a dark void, and throwing dozens of small blasts at the ground. the screen then negatizes, like luigi's smash, damaging all those put to sleep before firing an ominous wind attack, which flies down, and runs across the field to knock everyone flying.

Those are all the ideas i want to give. Whaddaya think?