Why a theme?
It would only cause less submissions.

I would say no theme, just a general yearly competition.
One can make catagories:

- 2D
- 3D
- Platform
- RPG
- Simulation.

and a grand prize for the best programmed game (in size and code).

in that way any competition will be appealing to any participants.
(I am also referring to the imho very annoying 'theme'of the Gamedev.net competition. Why has there always have to be theme. Themes tend to be boring and childish.
Just make your game and put it in my metioned cats.

b.t.w. I am also thinking of making my own competition, 2 times a year, only Object Pascal. The rest is free.
When I am serious I will seek Sponsors and make a site. (also Pascal Tutorials)