If you want an army that's primarily for display, then I'll assume hordes are out and that you want a reasonably low model count.
Obvious choices are Ogres, Chaos Mortals or Cavalry-heavy armies, so here are some ideas based on these.
Eastern Ogres:
Heavy mongol influence with lots of moustaches, fur trimmed pointy helmets, silk pants for leaders to show contact with Cathay, scratch-built scraplauncher based on a converted Cathayan repeater crossbow.
Chaos Mortals:
Pick your chaos power and go nuts! Or try an evil anti-ulric cult that worships a lesser daemon. Lots of blood-spattered wolf-pelts and wolf headdresses. Then practice your evil laugh for when playing Middenheim armies.
Cavalry:
Bretonnian - Cursed Mousillon army. Can be themed as either good (desperate to regain Mousillon's honour by cleansing the forests) or evil (in league with chaos/vampire forces). In either case the army can be represented as more than a little nuts, with lots of pelts, skulls and other un-Bretonnian trophies!
Empire - Knightly Order. Use an existing one or design your own. Supporting infantry can be themed as castle guards and hunters from the surrounding estates. WFRP books are a great source for more obscure orders or ideas to create your own.
Good luck with whatever you choose, and post pics so we can see how you do!!