The only reason I can think of it using the Just Cause engine is to provide a big open world that never really has to load. id tech 6 is stellar but I don't know if it can do open worlds well. Big set pieces for sure but nothing that would flow well without loading screens. I think if they can nail the feeling of movement and fluidity from DOOM then they'll have a pretty good chance, the other half is performance. id tech 6 is a brilliantly optimised engine especially with Vulkan that provided accessibility to a massive range of hardware with solid performance.