As someone has already mentioned earlier (sorry, cannot find the reference), theme should be chosen carefully and we should actually think about how would LD look like with that particular theme. There are thousands of participants, so we need to have thousands of different games. Imagine how boring it would be if we will have to play "Jumping Guy Shooting" thousand times.

So now imagine having theme like "One Room" (I think it's from last year) where you can do anything with one room. You can make horror game, or a farming strategy or use it abstractly, where won't be any physical room at all. This is a good theme.

Try to avoid too obvious or too shallow themes like: Like Dark Souls, Shooting, Arnold Schwarzenegger, Player is superhero, 1 vs 1,... and so on.

Theme should be more abstract and it should be able to be used for all genres, subgenres, and types of gameplays. So themes like: The other side, Life, Civilisation, Small World (from last year), Nature...

I know these examples are too short and abstract, but I hope you get my point.