Executive Meddling: What turned out to be a positive version. The original film was supposed to be a kid-friendly horror film. The producers felt it would work better as an adult themed horror film, and additional scenes were shot to up the gore quotient. And thus, a horror franchise was born...
What Could Have Been: The commentary for Back 2 tha Hood reveals that the original plan was to have the film be set on an island during Spring Break. However, the comparatively positive reaction to In the Hood resulted in the producers insisting on making basically the same movie again.
There were also plans to have the Bluewater Productions comic series crossover with the Warlock comics made by the same company.
Warwick Davis also claimed that a seventh film was going to be set in the Wild West, but the franchise is apparently (finally) dead.
The scene in the original film with the Leprechaun eating the Bland-Name Product version of Lucky Charms was filmed (with the approval of General Mills) with an actual box of Lucky Charms. The company later withdrew their permission as they felt the scene was disrespectful. Not only did they reshoot the scene, but they snuck in a blatant Take That at the company: Alex's Pre-Mortem One-Liner "Fuck you, Lucky Charms." For those wondering, the original line would have been The Tagline "Your luck just ran out."
Recently, Brian Trenchard-Smith, the director of 3 and 4 revealed that years back he had pitched another film that would have placed the Leprechaun in the White House.