Besides a few characters here and there, and inevitable new characters from certain franchises in the future, I really think we're running out of A-list Nintendo characters to add to Smash. In the past, Smash Bros. has relied on adding new characters to keep things fresh because Nintendo's IP is vast and there were always more characters to add. But now.....the list is not as big as it was. So how do you keep things fresh? I'd say do what other fighting games do: offer more customization.
That's right--CUSTOM MOVES DID NOTHING WRONG. Okay that may not be true but I LOVED the concept of custom moves, just was a little disappointed in their execution and limits and unlocking them of course. But the solution was not to get rid of them!!! The solution was to do it better.
Originally, when custom moves were revealed, I thought that you could manually adjust special moves--like you'd move a dial to make Mario's fireballs larger and slower on one end, or faster and smaller on the other end. That's one idea, though I think many characters would benefit from more personal attention being put into their custom moves. Ideally, most if not all characters would have at least some completely unique custom specials, like Palutena. Some were absolutely brilliant and tons of fun, like all of Mega Man's which were actual moves from his games, or Luigi's Iceball, Pikachu's Discharge and Thundershock, Charizard's Dragon Rush, Link's Giant Bombs, Kirby's Ice Breath, Samus' Relentless Missiles and Screw Rush, Zero Suit Samus' Shooting Star Flipkick and Plasma Dash--etc. etc. etc. I'm telling you guys, if you didn't have fun playing around with these moves then you were doing it wrong. There were a LOT of custom moves that I really preferred over their normal ones too. In fact, some did get implemented to a certain degree in Ultimate, like Samus' Super Missile being a little like her Turbo Missile now, and of course Palutena's Explosive Flame being a part of her moveset.
I miss custom moves so much!!! They would also resolve so many issues people have with certain characters like Ganondorf or Mario--want to play as Mario but you hate FLUDD? Use his original Mario Tornado as a custom special. Or maybe you want to replace Cape with a Cappy throw. Want Ganondorf to have his dead man's volley attack? Here you go. Samus could have her various beam weapons, or fire Ice Missiles. Olimar could have Rock Pikmin. Link could have square Remote Bombs, or his traditional bombs. Toon Link's Boomerang could be replaced with Deku Leaf. Donkey could have his Giant Punch replaced with his Coconut Gun, which fires in spurts. Rosalina could use Lubba instead of Luma. You could give every character a counter. Imagine the possibilities! The sky is the lim--no! There is no limit! Every character could be tailored to your preference and playing matches with friends or strangers would never be dull! Whether you want to surprise others with your character's customizations, or you want to stay on your toes by reacting and adapting to someone else's customizations, the need for new characters to keep things fresh would be reduced. Combine all this with more costumes and it's perfect. I mean they could even sell custom moves/costumes as DLC. So many options. So many dreams.....dreams that were shattered....broken, never to be repaired, because people did not understand the gift they had been given--alas, for a brief time only. Such is the way of things.