Scooby Doo vs. Death Note would be such an epic crossover. Here's a basic plot I just whipped up.
Part 1: A wave of deaths is sweeping the nation, etc., heart attacks, blabla, Kira. The Scooby Doo Detective Agency which has been in hiding for decades, led by Velma who calls herself L, has taken the case. All of Velma's deductions point to Light being Kira despite Fred's insistence that it's his old enemy Red Herring. Velma devises a plan to have a stand-in, Lawliet, approach Light as L. Stuff happens, ladies night, etc.
Part 2: Instead of Near and SPK, the Scooby Doo Detective Agency reveals itself, with Velma introducing herself as V, L's successor, obviously not wanting to give away that she was the true L all along. They arrange to meet just like SPK, except Light did anticipate that they'd fabricate a fake death note for Mikami, so he had Mikami hide a single death note page for emergencies and use that instead. Light says "V... This is my victory." Velma, Shaggy, Freddy, Daphne, and the entire police force except for Light all die. Scooby fakes his death (animals aren't affected by the death note), and Light walks out, thinking himself victorious. Several minutes later, Light gets a distinct feeling of being followed... Nah, it's just my imagination, he thinks. Then suddenly a dog jumps on him and pins him down! The last thing he sees is Scooby Doo's teeth before they rip his throat out. Then Scooby Doo, his vengeance complete, with Kira's death, he bays forlornly to the moon. "SCOOOOOOOOBY DOOOOOOOOBY DOOOOOOOOooOOoOOoOOooooo. .."