If I had a time machine, there's two things I'd do:
A. First off, I'd go back in time, to around April 2013, and leak off what's going to happen at E3 2013, as well as Rosalina in December. I'd also mention stages, ATs, items and such. My only proof would be release dates of certain characters, like Sonic, Luigi, Olimar, Peach and such.
B. I'd go to the future, find out everything about the game, and then come back to current time and give away everything wrong, like giving opposite characters (a Prime Hunter instead of Ridley, Tiki Tong instead of King K. Rool, etc.). Because I leaked everything right before, everyone will trust me and freak out for no apparent reason besides my cunning.
C. Laugh and then make it so I reveal the entire roster a few days later, joking around with them.
EDIT: I'd also probably go to the future and deck out my time machine to switch between TARDIS and DeLorean mode as well, probably take an almanac from the future, leak the next Smash Bros. roster, bring the cure for cancer and AIDS, murder Hitler and Osama bin Laden, prevent Lincoln from dying, buy all the old gaming consoles with their respective games, go to Disneyland each year from 1955 to 2055 to see the changes made, find alien life and go to the moon with Neil Armstrong. I'm a big dreamer.