Tetris Block. Imagine it using moves inspired from Tetris' gameplay, like changing form (just its hitbox, not as drastic as Brawl's Pokémon Trainer), summoning small blocks with the same shape as its current one to complete lines and do a powerful explosion, overlapping S-blocks under it to create a staircase as its recovery, or having a dair based of the fast drop mechanic. It could also have moves inspired from its shape mechanic, like having its Smash attacks based on projections of its current shape. Add cute eyes to its design so it will not feel like a soulless thing.
Well, at least a Tetris Block like this wouldn't be a
complete joke character, actually the concept would be like Wii Fit Trainer, using a character from a series which does not rely on a plot and adapting gameplay elements to Smash Bros. even though those games have nothing to do with fighting, as well as representing an important part of Nintendo's history. Sure, Nintendo does not own Tetris (actually the company which own the franchise is... the Tetris Company
) but the Gameboy version of Tetris is the most famous version of the
most famous game of all time and it helped a lot to sell the system at the time, and thus deserves a playable character. Though at least it's good that Tetris is already acknowledged in Brawl and Smash 4 with cool remixes of A and B type musics.
For a joke character for the sake of a joke character, my favourite choice would be Magikarp, arguably the best positioned character for such a role when you think about it, when taking account its huge notoriety based on the fact of being bashed for its weakness and sold by a fraud who tries to persuade the player that he made a good deal. Also, Magikarp learns exactly four moves in the Pokémon games (event moves Bubble and Hydro Pump don't count), so it Smash special moveset would be 100 % true to its source material, and its Final Smash would be a 200 base power Flail (you know, when a Pokémon uses Flail with only one HP remaining). It would be also the first fish fighter in the Smash history. Just imagine how the internet would react to its reveal or how amazing a Magikarp beating Master Core would feel.