Guests who have park-to-park admission will be able to take the Hogwarts Express between the two parks. The Hogwarts Express has windows that are made of screens, so instead of seeing some Orlando roads as you take the train between parks, you will see the English countryside as well as magical creatures from Harry Potter. Everyone's dream to take a ride to Hogwarts can finally come true.
The London waterfront is what you'll see first, which hides the new section of the park like in the films. You'll see Leicester Square, the Knight Bus, and Grimmauld Place before entering Diagon Alley through The Leaky Cauldron, which will be massive in size. New shops besides The Leaky Cauldron that are featured will be places like Weasleys' Wizard Wheezes, a larger Ollivanders Wand Shop, Madame Malkins Robes, Magical Menagerie, and Quality Quidditch Supplies to name a few. Knockturn Alley will also be included, for all muggles to get their fix of the dark arts.
The star attraction will be Gringotts Bank, which houses the new ride "Harry Potter and the Escape from Gringotts". Likely a roller coaster with brand new technology, its unknown exactly how the ride works, only that it will be like the escape seen in the Deathly Hallows. A fire breathing dragon will be perched on top of Gringotts, and will actually breathe fire.
JK Rowling has been involved in the process, and helped introduce brand new shops and characters that weren't featured in the books or films. Take a look at a teaser video of the expansion: