For starters, this time around, Traveller's Tales is incorporating an open-world approach to the Lego Batman universe. While the core story will be pretty linear--involving an evil plot concocted by The Joker and Lex Luthor--there will be opportunities to go off the beaten path and explore the city of Gotham. The entire city is yours to discover. The story may have you visit certain locales throughout the adventure, but if you want to go back and check out certain areas, you're more than welcome to do so. Of course, some areas will be accessible early on, but others will require certain items, specifically suits, for the characters to reach them.
Loved Lego Starwars and got hooked for a long time, but then they started using cheap mechanics like the very zoomed in perspective most of the screens allowing enemies to shoot at you from off-screen. But if they do co-op right this might be the game that gets me re-addicted to the series.