Stage: 25m

Perhaps the most famous first stage in video game history, 25m is something anyone can tell you what game it's from. We already have the 75m, so why not add the more famous stage to Smash as well?

Assist Trophy: Toad

I think before Toad becomes a character, he needs to be an assist trophy. He could just jump up and down saying "(insert character here) the princess is in another castle" and used as a distraction.