There is only oe true answer to which character should be the next Smash Fighter, and that is...
Mr. Mime.
His neutral special could be a chargeable barrier-move that creates a wall neither you nor your enemy can pass through, with charging it increasing its duration and durability. Mr. Mime's attacks could influence it similarly to how Pacman and Villager can hit their hydrant/tree to benefit themselves, except Mr. Mime could push his barriers forward, to smash opponents on the opposite side of it away and to reposition it, or turn it by attacking its upper or downer end, launching people hit by the edge of it depending on how you turned it(if you turn a vertical barrier by attacking its upper end for example, it would swipe down to the enemy like a tennis racket, meteoring them. Likewise, Dairing of a horizontal platform's edge allows you to swipe the enemy below your platform off the stage with great horizontal knockback).
His Side Special could be Double Slap, which is a mix between Jigglypuff's Pound and a directional Smash attack that, while extremely weak by itself, can be used to advance with the barrier extremely quickly and upset the barrier at basically any time. It would pause his vertical momentum and help him recover when he needs it, however, the attack would stop and put him into helpless mid-air if he'd hit nothing with it(neither character nor barrier), similar to how multihit-moves in Pokemon seem to work(missing one in the chain ends the chain immediately). If all 5 hits connect in mid-air or the combo is interrupted prematurely, Mr. Mime enters helpless state anyway.
Up Special could be Encore, which is a move similar to Jigglypuff's Sing, except it will force the enemy to repeat the same move he just did for a second or two, with the effect's duration increasing the higher the enemy's percent is(however, directions of the move can still be influenced, you can turn Falcon Punches, and recoveries don't have to carry you off-screen). Encore could be used for things as simple as punishing opponents for whiffing extremely predictable moves, to gimping your opponent by locking them out of currently useful recovery methods(woe to the Mario that gets locked into Cape just below the ledge).
Down Special would be Recycle, which is basically kinda like Peach's Turnips except with a chance to recycling the item you last had in your hands(including enemy projectiles which can be picked up and thrown, however projectile items will spawn without any bullets and are only able to be thrown away), Leftovers(which can be dropped and eaten for a minor health recovery), Toxic Sludge(which acts similar to a sticky bomb in that it sticks on walls, enemies, even your barriers, but unlike them will gradually inflict heavy damage over time instead of exploding, and heavily damage you as long as you hold it in your hands(and your barrier if you let the sludge stick on that, as well!) and Leppa Berries(which, when thrown, inflict no hitstun and 1% damage, but when held allow you to use a special, even when helpless-falling or during ending lag of another attack/special. This not only allows you to use 2 double-slap combos in one go, but also allows you to create and double-slap a barrier in one go while off-stage even without a doublejump, giving you an even more amazing horizontal recovery). Recycling the last item you held will be the most likely outcome when recycling, but with each successful recycle of that item, the chance decreases, slowly favouring Leppa Berry, ludge and Leftovers over the item you held last. Out of those 3 items, Leppa Berries are the most common, followed by Leftovers, followed by Toxic Sludge(unless your last used item was one of those 3, as this chance stacks with the default chances for the items).
Finally, one of his taunts and one of his victory poses would literally just be him staring at the screen at the player, piercing their soul with his strangely creepy smile.