Smash Run looks pretty cool. I always liked the sort of challenges where enemies are constantly after you and you must survive. When said enemies come from many different Nintendo games (plus Mega Man, Sonic and Namco arcade games), it becomes amazing. Run the gauntlet and collect loot! It's like plundering an enemy base...
I actually like how they brought back some SSE enemies.
Hope we have boss fights on the challenge doors. And let the bosses be varied as the enemies. Some could return from previous Smash games like Master Hand, Crazy Hand, Porky, Galleom or Petey Piranha, but there could also be more bosses like Eggman who could also have several different battles with his many robots, Underworld bosses like Twinbellows or Pandora from Kid Icarus, dragons or foes like Ashnard or Black Knight from Fire Emblem, Zelda bosses that are so many, Bowser Jr. on the Koopa Clown Car...
There are lots of possibilities for bosses that would be one hell of a challenge on a Boss Rush mode like Brawl.