To be built where the Universe of Energy attraction existed.
Melissa Valiquett, Epcot Vice President, described this as a roller coaster at the Epcot 35 celebration.
The steel and sit-down categorization are just default settings at this time. They have yet to be confirmed. It is interesting to point out, as of this writing, all Disney coasters are steel, sit-down coasters.
At the D23 Expo in Japan (February 11th 2018), Bob Chapek, Chairman of Walt Disney Parks & Resorts mentioned that 'Guardians of the Galaxy' will become one of the world's longest enclosed roller coasters. The current longest enclosed roller coaster is 'Temple of the Night Hawk' at Phantasialand in Germany.