Original plans called for "Cartoon World" with Dr Seuss but also D.C. Comics characters, Looney Tunes, and even for The Simpsons. Six Flags renegotiated for Looney Tunes and D.C., effectively locking those characters out of the park while The Simpsons and Gracie Films were not willing to play ball.
During development the focus of the park shifted as Jurassic Park The Ride opened with critical acclaim in Hollywood and an islands theme was adopted. Marvel Comics (seen as second rate at the time) signed a lifetime deal with the park as did many smaller cartoon characters. Jurassic Park made its home at the new park as the star attraction and a new original land based on myths and legends was developed (no it wasn't the same plans Disney had for Animal Kingdom).
Construction started in 1997 for CityWalk and Islands of Adventure for opening in 1999. Islands opened with 13 rides (including the Island Skipper Tours) and three shows. In the intervening years a small coaster, a flat ride, and the much delayed Seuss Troll ride opened but none brought back the crowds after the September 11th terrorist attacks. IoA and USF languished at around 5 million guests a year, each, for nearly a decade. Then a certain wizard came to town and changed theme parks forever.
Islands of Adventure is and has remained one of the best parks in the country. While we take it for granted now the combination of thrills, theming, and design was never before seen. Remember IoA was the first park to receive a full, custom soundtrack. Nothing shows the excitement and wonder better than Sean Flaharty's pre-opening day video of the park. In the video Sean shows us the famous IoA model, construction footage of the park, and a pre-opening day tour. The energy and amazement is palpable and quaint, with many guests not knowing what each attraction is.
Other sites have more great Islands of Adventure info. Theme Park University has a series on the never built original concepts for the park and Theme Park Insider dives into the history of Potter in the park.
This year for the 20th Hagrid's Magical Creatures Motorbike Adventure opens and a new Jurassic Park experience is on the horizon. We cannot wait to see what the future holds for this amazing theme park.