From my personal experience, there are still some restrictions. You cannot effect the physical plane from the astral plane. Now some of the laws of physics still apply, such as Newtons Laws of Motion. And that energy cannot be created or destroyed.
As for fighting in the Astral. There are those who are stronger than you. There are many beings on the Astral. Demons and Angles reside on that plane, as well as deities.
As for imagination, well in the Astral there is a lot of visualization involved, as for creating your own reality, you can in away, but not really.
The Astral is a dimension that is pure energy. From my personal experience, there Is already a reality that existed their long before humans were even in existence, keep in mind I am speaking from personal experience, which can Also be converted as my opinion, for as far as the Astral Plane goes, there is no solid proof or evidence.
If there is I would like to see it.