It seems like Super Smash Bros. for Wii U will likely be the last game we'll ever see in the series from the creator Masahiro Sakurai. In an interview with Game Informer he says that there might be more games though, but those will be made without him.
"I can't positively declare there won't be [another Smash Bros. game]. With both Melee and Brawl, I made those games with the thought that there wouldn't be any more sequels. Thus, I really can't deny the chance for another. However, as for myself, I don't think there will be."
He went on by explaining the sheer scope of the game and how a sequel would need to get more content:
"Our intention [with Super Smash Bros. for Wii U] was to provide extra merits to the game which go far beyond the sale price. In terms of scope, and in terms of sheer number of characters, we went beyond our limits long ago. And yet, if we cut the number of fighters or modes in a future game, I'm sure there would be complaints."
And that amount of content is more than Sakurai could handle and he says it would be like tightening the noose around his neck. Someone else might do it though, but Sakurai is skeptic:
"You could say that all the effort in the past to stretch out, keep pushing myself, and provide all these extra merits wound up tightening the noose around my neck in the future. That may seem like it contradicts my personal desire to keep giving gamers as much as I can, but I don't see any easy answer for it. And yet, despite that, I also have trouble picturing someone else taking my place and providing all this value-added content without me."