The plot is simple for now: Bowser has kidnapped Peach and Mario swears vengeance. The interesting thing I came up with is that the characters are aware that they are part of a game, ala Banjo-Kazooie, so Mario will speak to the player throughout the quest.
The hack features:
No ExGFX (Doesn't fit here because the main game has none and it doesn't make sense to me to have the prologue have ExGFX and the main game have none.)
Slightly edited SMW graphics
Brand new levels
Crappy custom boss idea :P