I'm not sure if this would be too drastic a change to include at this point but i think the rapid jab finisher would be a nice addition. I've included my idea of what the finishers would look like for each character and how this can be balanced.
Diddy - bucks with both feet after rapidly attacking with his tail. knockback is akin to his fair but it doesn't go as far.
Falcon - brings the hand he isn't punching with forward in a mini falcon punch. deals burn damage and has good horizontal knockback.
Fox - steps forward after rapid kicking then executes his 64 utilt. can follow up with uair. (pseudo up throw up air)
Falco - since he's been attacking high he finishes low. he crosses his wrists then separates them at his opponents feet. knockback puts them just above his short hop.
Shiek - same thing as in smash 4. good to follow up with fair at low percents.
Link - stabs forward like marth's brawl shield breaker or tinks dash attack. knockback is like 64 link dash attack.
Mewtwo - while rapid hitting his offhand produces a small shadowball. he reaches forward with his off hand like in his taunt and connects with the ball. knockback and damage replicate foxes shine exactly except with purply shadow damage.
Pit - after rapid hitting he yanks the sword/bow back quickly and stops spinning it. deals poor knockback but decent damage.
Kirby - after jabbing he finishes with fighter kirbys rising upper. can be followed up with uair or fair depending on the character.
Metaknight - finishes his rapid with the same attack as his 3rd hit from the tilt. everything copies over.
Dedede - after spinning the hammer he pulls it torwards him which yanks the opponent in his direction. good to follow up with a grab at low percents. higher percents result in knockback similar to docs dtilt from melee.
Game & Watch - after pumping whatever that thing is, he pumps one more time for a massive multihitting cloud. good on shields and stuffs sloppy recoveries. not appropriate for children.
Balance
For balance the jabs and finishers would need some tweaking so they're not stupid safe. First and foremost, rapids should be repeatable only twice after the initial cycle of rapids. After the third cycle (rapids have a repeating pattern) of rapid jabs you WILL perform a finisher if you stop mashing. With the first/second cycle you have 2 options; continuing jabbing in hopes of trapping the other character or dropping the rapid. Performing a finisher with all characters will inevitably result in a longer cool down than if you had just dropped the rapid jab. While dropping the rapid still gives the other character a small frame advantage, you will be able to act much quicker than if you tried for the finisher.
On the defensive side, the shielding player will have to predict whether they will go for the finisher or drop the rapid. Predicting incorrectly will either result in getting trapped or grabbed. Depending on the character that is rapid jabbing a correct prediction might result in a shield grab, up b oos, usmash oos, wavedash in/out or even a buffered roll to escape finishers which might be slower than others. If you get caught in a rapid there's still hope. Depending on the character you should be able to DI into a position where you'll still get hit by rapids but if they try and finish they'll miss and leave themselves open.
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