A+B.
This would open up a veritable world of possibilities, anything from alternate basic jab (see Falcon example) to giving the much desired opportunity to show off some of the other items from the character's universe by incorporating them some small, tasteful way (Samus example), alternating their playstyle entirely (Link Example), or even just another simple button input for a 5th special move (Luigi example).
Link, who rarely uses things such as his Hammer, Power Bracers, Iron Boots, could have those slipped in here. I can really imagine a playstyle of Link revolving around a combination of Iron Boots and Power Bracers if some delegation likewise to my proposed attacks does not go to another player. Link, hitting the A+B, Dons his alternate set of items, particularly devoted to his heavier armament: Hammer, spinner top thingie, Ball and Chain, Iron Boots, Power Bracers, the Fairy Sword from MM/Giant's Dagger from OoT, what have you, the opportunities are endless. Ness, different set of PK moves - Mother/Earthbound fans help me out here -; Marth, alternates between stances and styles (please refer to ARMA (Association for Renaissance Martial Arts) to get a sense of the depth of stances and strikes that align under the banner of 'fencing', which is the style that Marth is mainly inspired by).
In addition to Captain Falcon's staple, awesome, and leisurely 'Gentleman', he can also preform the more close ranged 'Good Sir': A horizontal elbow swipe his forward left arm, a step forward and a vertical strike (to the collarbone to give you reference of angle) with the other, and then a tight uppercut with the alternating arm, the last giving a little bit of that 'B attack' flavor of Falcon with some fire effect and a 'HMPH' Kiai.
Samus has many items left untouched, I would personally love to see her Side-B replaced with her Speed Boost, and the A+B replaced with a toggle/switch between various different beams and missiles, or just transfer the missiles to A+B.
Luigi needs a button for his Poltergust, and I want at least one of the Mario Bros. to keep the tornado in their special, A+B could be an effective solution, this I feel is the most probable because every character in the cast can derive the same amount of depth with such simplicity, none would be SO underwhelmed because of the switching properties of others.
Edit:
Yes, so much yes, back and forward taunt should be different due to their static nature.
This is genius and hilarious in its possibilities.