Base Terraining - Some one will draw a very basic outline and you have to make that into a terrain and then add to it.
Post Civilization- A futuristic town, all human beeings are dead, and the whole town is overgrown with big trees, plants, etc.
Elements - Basically like the four seasons terraining contest. You make a base terrain and then change that terrain to represent each of the 4 elements. Technically you'd be making 4 terrains but they are all just edits of the base.
Paired Terraining Self Explainatory. The problem is that you would need like 14 participants (7 entries) to make it a good contest. But it might still work. You never know. Maybe you could combine this with one of the other themes.
Battle scene You would make two armies attacking a castle, fighting in an open field, etc.
Cataclysm - Make a terrain of a reknown landmark of the Warcraft lore (or just some place in general) and make it how it would look ravaged, and maybe even have two shots, one before its destroyed and one after.
Blue- Make a terrain that shows the color blue.
Big - Just terrain something big, like a huge robot destroying a town, or a big mountain that has LOTS of small villages and even kingdoms, or countries on it.
inner-city - terrain the inside of some capital city
There you go Keiji, The suggestions from this thread. (I probably missed a bunch, I just scanned through it quickly) Now can you just make the poll. It doesn't matter what the theme is, you will always have atleast one person who doesn't like it. Therefore we could argue it forever. So a poll will hopefully get this contest started.