I get the feeling that Brawl was designed to be the last. I mean, I could be wrong and, even if I'm not, that doesn't prohibit a mind change from nintendo but just look at the game.
They spent more than twice as long on it as any other smash game, gave us a stage roster that took up more than the whole screen, made a story mode with a 3 hour final level and the previous boss character in chains, not to mention a boss with 1 hit KOs and the supposed destruction of the nintendo universe. They threw in fan favourites from other companies (though that was intended for Melee so probably doesn't count) and a large chunk of the story revolved around a new character who wasn't even from a video game (R.O.B.). And then we've got the new theme. It sounds epic, the meaning sums up the entire series into one epic legend and it's even in latin. And then they made the game focus more on ariels. Everyone knows air combos are amazing.
Yes, we all know the gameplay changes were a mess but it is still downright glaringly obvious that they pulled out all the stops to try and make this sash as epic as physically possible, definitely hinting at it being designed as the last.