My game store is having a diorama building competition. I entered, but I need some ideas on what to do.

I don't want to go out and buy boxes, I'm going to limit myself to old bits/models I have on hand.

Here's some bits I have

Death guard: around 20 unbuilt poxwalkers, a few odd bits from starter sets, some nurglings, a parted out bloat drone

Admech: vanguard sprues, an old dune crawler, assorted knight bits(mainly guns/face plates)

Orks: leftover Boyz, meganobz, nobz, and some grotz

1k son's: tzangor bits, rubrics, sorcerer's & tzangor enlightened

Some cool pair ups I thought were admech vs orkz. Maybe the admechs crawler got disabled and the orkz are closing in. Or the factions are fighting over some knight scrap.

Another idea is a lone loyalist marine assisted by a few skitari are cornered by a horde of poxwalkers rising from the ground.