Seeings as it is "the year of Luigi," the green-hatted Mario brother was an obvious inclusion.
Nintendo has deemed 2013 as "the year of Luigi," and has spearheaded multiple projects that bring the more cowardly of the Mario brothers into the spotlight, such as New Super Luigi U. This year also saw the official announcement of a new Super Smash Bros. game, and you just couldn't call it "the year of Luigi" if he wasn't included in Nintendo's mascot fighter.
Luigi has been in every iteration of Super Smash Bros. since the original N64 title, so while his inclusion may initially be a no-brainer, you have to remember that director Masahiro Sakurai did state that wanted to remove characters that didn't bring anything unique to the table. As Luigi plays quite similarly to Mario, there was a legitimate concern that he might be cut.
Nintendo has released a slew of screenshots of Luigi doing his thing in both the Wii U and 3DS versions of the game. Luigi joins Captain Olimar, Mario, Donkey Kong, Pit, Link, Samus, Kirby, Fox, Pikachu, and Bowser as returning characters to the series, and will be joined by newcomers Mega Man, the Villager, and Wii Fit Trainer.
My only worry is, now that Luigi is confirmed, what does this mean for the vastly superior Mario clone, Wario? Who else do you want to see return in the new game, and who would you not shed a tear for if they were cut?