Well I think your ideas are pretty sussed and will look good, the only advice I can give is 'realism', make the scene look plausible. You see so many awesome looking heroic posed displays that in a strange way also look boring because of that. One of the best ways is search the net for real photos and then '
40k' them. WW2 Stalingrad, Ardennes and Nordwind Offensives and anything from a German perspective in 1945 makes great material. Perhaps not the best for originality but screenshots of Dawn of War 1 set in ruins or winter e.g. winter assault is an easy way to get ideas.
Also giving the display a message theme is good too. Are the Guard holding a strong line? Have they nearly been overrun and Space Marines turned up last minute? Are they preparing an assault from their trenches or are they looking to pull out? Are they ready for battle or have they just been through one? etc etc
Best of luck I am sure if you managed to come second last time, you have it in you to come first this time