When Shovel Knight came to Kickstarter it's amazing retro graphics and fun premise catapulted it to much success. So much so that they hit each stretch goal they set. Yacht Club Games decided to ship the main base game and after the initial launch bring a lot of the stretch goal features later. The in-game stretch goals were the ability to play as 3 bosses, gender swap mode, challenge mode, 4 player battle mode, ability to play all bosses in battle mode & the Linux & Mac ports (already released).

From what I imagine is the first of the playable boss modes, Plague of Shadows is coming as a free update and it looks to be a entirely separate story and play experience than the base Shovel Knight game.

"Shovel Knight: Plague of Shadows is a free expansion for Shovel Knight, a sweeping classic action adventure game that won over 70 Game of the Year Awards in 2014. You play as Plague Knight, the villainous master of alchemy, in a quest all his own. Using his maniacal scientific mind, Plague Knight is seeking to concoct Serum Supernus, the Ultimate Potion. However, to retrieve these ingredients, he must extract them from his former allies… the villainous knights of the Order of No Quarter! World class platforming, music, storytelling, and art all return in Shovel Knight: Plague of Shadows; an explosive quest with a new play style, new story, new bosses, and more!"

http://yachtclubgames.com/plague-of-shadows/