Zero Gaiden will be set in the planet Bram, while Ultraman Saga will have battle on earth and it's going to feature miniature again according to the japanese wikipedia.
It's not confirmed yet whether the battle on earth is going to be short or not. I'm hoping the final battle will be Zero coming to earth and fighting with the others instead of Zero bringing the others to space from earth.
Well I can't blame them because for one it frees up the fight scenes not having to be on a set with building models. Also I don't think there's been an Ultra before that operated exclusively in space so that does kind of give Zero some distinction from other Ultra's.
Though his finally having an adventure on Earth would be interesting.