For me I would Lyn from Fire emblem to have an Iaido gauge that fills up for every hit you get on the enemy when it fills completely, her stats gets boosted especially her damage output since she'd be a fast hitting combo speedster who lacks power.
By pressing B, you can preforms the same move as she does in her AT, automatically flash stepping to the nearest (or even through all) opponents and performing a critical strike upon the opponent for high damage and knockback you can even slash through all 8 of them (assuming if 8 player smash returns) if your lucky but the more enemies you cut through this way, and the further your enemy is, the less knockback there is but damage stays the same.
Instead if you slash through most or all your enemies or if they are further away, an very quick animation will show your opponents stunned stiff while Lyn quickly sheaths her blade with a 'click' and the enemies fall to the ground taking high damage similar to Snake's sleeper hold throw (forgot which direction) if timed right enemies can power shield out of it so this move is best done after a combo.
Another idea (a bit more difficult to implement I admit) is she could use afterimages instead after the gauge fills a little with a couple attacking along side her (number of afterimages depend on how full the gauge is max 3, 5 if the system can handle it)allowing her to attack faster and extend her combo kinda like an Astra ability in FE enemies can attack the afterimages to stop them or attack Lyn to cancel the ability altogether.
Or to make it less complex, filling the gauge can activate Astra and the overall speed and the damage of her attacks and movements increase dramatically.
Another characters that could have cool mechanic is the Bo Staff wielding Krystal from Star Fox, for those who don't know she's a telepath. So why not add a mechanic that helps her avoid moves like she's reading the minds and movements of her opponent.
"Reading movements? You mean having visions like what Shulk does?" Quiet you. No, not even close. She would have gauge similar to the staff gauge in Star Fox Adventure except divided into ten segments to fill the gauge, you need to attack your opponent three hits with standard attacks fills one segment (even if you hit with a smash attack it still counts as one) one special attack fills two segments, and a KO fills 5 .
When the gauge is full you just hold shield and whatever attacks that comes land or air will automatically miss Krystal, it even cancels out end lag and shield stun from Krystal's moves and allows her to punish.
The catch is you can only dodge three moves it could be in a row or separate occasions but you can only dodge three moves and only when you hold shield (final smashes not included). Also the gauge increases her overall damage and the elemental powers of her staff (B: Fire Blaster, Side B: Ice blaster, Up B: Rocket Boost, Down B: Ground Quake) so you have to decide: build up your gauge to auto dodge oncoming attacks, or use the gauge to annihilate foes with the forces of nature.
, was my favorite back in Brawl. I liked Ivysaur and Squirtle better than Charizard, so I didn't really like that they brought back Charizard...
