Could I pick your brains??

First. I'd lose the tree. It looks like a toy tree or Christmas decoration and adds nothing to the composition. The upper branches are way too thick.

Next, I'd reposition the SPG. The barrel has a very strong horizontal line which brings the viewers eye away from the focal point.

The truck also points away from the focal point.

The position of the officer and the corner of the house are really good. The house is "aimed" at him and makes you want to look at him.

Try putting the SPG to the left of the house (as you look at it) with the gun pointing to the center. Next, move the truck to the other side of the building driving towards the center of the base. Again, leave the tree off. Maybe add a road leading from the middle of the curved section towards the corner of the house, making a Y intersectoin with the officer more or less centered in the Y. Add small shrubbery or very low ruins to the areas of the sides of the leg of the Y.

Mark

I am not sure how much additional 'stuff' you have to put down, but to me your base looks kind of big.  There seems to be a lot of empty space, particularly between the vehicles and the building.  One thing you might want to consider - if these guys are taking a bit of a break, why not move everything in tighter to the building and have them sitting around the entrance.  That way you might be able to make the base a little smaller and reduce the emptiness.

Looks like this is going to be a nice little scene when you get it all done.