The latest update on the Super Smash Bros. page on Miiverse takes a look at how the Classic mode works.
Using the Nintendo 3DS version as an example, Sakurai highlights a familiar mode in the upcoming game, where players progress through a set of stages toward the final battle. To increase the challenge and rewards, players can pop gold to tip the scales.

The more gold you put onto the Fiend's Scale, the harder the battles will be. In return, your rewards will get better and more abundant. The best way to earn gold fast is to win Classic battles on higher intensity.
Sakurai also confirmed the appearance of series favourite, Master Hand, but also teased that something else could be waiting at the end "if you raise the intensity...?!"

What do you think of the classic mode in Super Smash Bros. for Nintendo 3DS?