Okay. I'll admit up front I thought this up, someway, somehow, while doing repetitive, mind-numbing counting of stock at work last night for retail inventory.
Anyways, Forrest Gump gets kidnapped by Bowser shortly after becoming a ping pong star. Seeing a mind that he could manipulate to do his bidding, Bowser gives him a mushroom to make him as tall as the koopa king, and places Forrest as a boss for Mario to fight in the midst of Super Mario World. Forrest now has a special ping pong paddle which he can use to launch bouncing bob-ombs at Mario that will either explode after a few seconds or instantly explode on contact. This, coupled with his natural speed and agility have Bowser confident he can take down the plumber.
Bloodlust is on for Forrest, and Mario knows he's gotta make it past this Tom Hanks lookalike one way or another.