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BaganSmashBros
I'd consider both an inability to attack up-close and the ability to allow attacks to pass right through her to be unique mechanics, but if you mean strictly that which works outside of the Smash standards:
Lucario's on-hit-cancels are a completely original ability, as well as gaining XP from hitting opponents which can then be spent on super-versions of any of his specials.
Olimar's Pikmin budding may have been in Brawl, but it had no effect and only became a proper mechanic in PM.
I'd consider Solar Beam to count if Shulk's stuff does, considering that Ivysaur can gain charge from a B-move (that also heals), from sweetspotting some normals (which also heals her and puts a flower on the apponent), and even pummeling (which also heal).
And on the topic of Lucario, there has yet to be an implementation of a Rage mechanic in Smash that didn't suck, something which I believe is appropriate and possible for
Ridley.