to get SCXE 2.6, just search google for SCXE 2.6 Download.
You should be able to add new units, which you can completley customize with a built-in unit editor that's kind of like a 3d modeler, and when i mean completley i mean you can change every single aspect of it, terrain should have a huge range of heights like the wC3 editor except more so, maybe like 255 even and a option to scale the terrain however much you like, and you should be able to put anything anywhere, like a bridge between 2 mountains, terrain should be truly 3d as in you could make a tunnel through a mountain and if you played it the mountain would become semitransparent where there were units under it, and you could also make things like caves, and the ability to plave wells or fences or whatever that can be shot at to destroy them but like units you can change how much health each part of the wall has (these walls should also be buildable in-game), individual unit stats should be editable like master virus said, and there should be triggers to change terrain, lyou could either make another mini-terrain in the same map but not on the map, and you could have the trigger change replace what is at a specific location with that terrain, or you could specify exactly what it would be changed to with the trigger editor using a scripting language, locations would be much fancier: they could be "painted" which means you could set a brush size and paint it on the map and it would make it part of a location which you would select in a drop down box with one of the options being "New Location", if you picked that it would pop up a box asking for a name, or you could use a line tool to draw a straight-edged shape, or you could use a combination, and of course all the other neat features that I can't think of right now.