Here is a consolidation of HOTH suggestions as well as a few extra bits to consider (some of which have been already pointed out):
(1) consistency of backgrounds - like someone mentioned, this is going to be a hell of a lot harder than adding Yavin in the background, but still something to attempt. Also, is that laser crooked on the top AT-AT in the upper right?
(2) add squadrons of snowspeeders in the background doing strafing runs or fly-bys, not just for the visual aspect but also to account for the large group we first see when they come over the hill and fly over the trenches. Could also consider showing the AT-ATs shooting down more snowspeeders, as seen outside Luke's (or other snowspeeder pilot's) canopy as well as other shots.
(3) more laser fire from the AT-ATs - this may not need to be done, and even so it may be a matter of adding a few extra pulses or add more bolts coming from the side turrets (as mentioned by ImpFighter). Maybe if a stronger bass track could be added to the laser fire to give them more oomph in the sound mix.
(4) a lot of shots need more depth in the background elements, via Vaderios' last set of recreations.
(5) also need consistency of the ground elements too - in the falling AT-AT shot, the snow is broken. In the second shot, there is wind blowing across the snow.
(6) you could simply make the radar guns into smaller ion cannons (not make the dish-guns spherical, but just change the energy glow on the dish to blue) and change their laser bolts into the electrified blue bolts as seen in Vaderios' ion cannon shots a few pages ago. Maybe you could also show the effect of an impact where the ion blast seeped through the armor and a leg stops moving or begins to stutter or slow down - Vaderios: I was wondering if you could create a mock-up of this concept with the above pictures.
(7) also consider showing damage from the various ion/laser blasts from the rebel cannons, as well as show the small arms fire of the rebel soldiers firing at the AT-ATs.
(8) all damaged or destroyed AT-ATs need to be added in the background, as well as possibly the other 2 flanking AT-ATs seen in the prior "routing of the rebels" shot. Could also consider adding more flames and smoke to the destroyed laser turrets and maybe even bodies of rebel soldiers.