The difficulty in picking the themes is you have to make sure it's something that all players can understand and contribute towards. You've also got to make sure you give the theme enough room for decent improvisation and customisation within each "plot of land" without compromising the cohesion of all the builds.
We tend to stick with terrain builds because it gives an easy template to follow with the objective of making your plot fit in with those next to it, while also leaving you with lots of space to do something interesting to differentiate your work from everyone else.
I think another event exhibition would be a good idea, though. We haven't done one for a while.