Going down the route of Devils Advocate.
To a man There's not one of you who couldn't conceive a cunning plan with in five minutes for any of the topics in the list. Being honest you've more than half a plan already if you're proffering a topic.As it more than likely to fit in with a current project.
So why not take this into account.
Simply when you enter state a period/genre you intend the finished build to be use in. Once the competition is over.
It'll give you motivation and diversity to the competition.
More over if you can't finish In time or have to pull out of the competition. Then your not left with a dead build with no relevance to your projects. Rather something that remains relevant and can be picked up again when your able or just at a slower pace than competition speed.
That way you've no dead or lost hobby time, and your not sat hoping the build choice will have relevance to your hobby space.
How to add the challenge?
Ignore all the usual choices and list's and use something more abstract such as one of the following. A line of poetry, a song lyric, a paraphrased quote from a movie ,or even a phrase.
It does force you to use 'the little grey cells' more than relying up on your crafting abilities.
How to choose the theme?
(With the exception of those wish to enter) why not have the modes privately proffer their choice for a lyric etc. To the competition organiser. Who then choose from the compiled list.Randomly if need be.
Of course as always having the theme remain hidden until after the closing date for entering. Is much more of a level playing field.
Just a musing