Well, if those DLC are actually for testing waters, that makes sense: a character, a stage and a game mode. Once they figured out how much money/work those single thing cost, and see how they are received individually by players, they can easily start to think about either working on paid DLC or not.
After all, game are investments, and paid DLC too, right? If making one new stage cost them too much money and nobody buy it, they will (of course) stop making new stages, paid DLC or not. If, on the other hand, the new character gain a lot of success and people seems interested, more character will come.
By the way, Sakurai's crew is not working on a single character, but on two: one for the 3DS, one for the Wii U. If you think about it, that's most likely why Bowser Jr. was about to get cut. He got an entirely new moveset, and 8 different character models (similar of course, but still needed different 3d models), so basically for this newcomer they worked on 16 different 3d models, animations and effects at once, on top of new moveset. And twice, since the game got two versions.
A lot of work for a single fighter, huh? Considering the game has 51, it doesn't surprise me we got Lucina and Dark Pit...