I would be interested in this, but I would be adverse to a 'topic' or 'theme'. These are longer works, with much more of an investment, and in my opinion, themes are better left to short stories. Unless we did something like a shared world, or some shared setting, which might work. But then genre might be a concern for some of our writers.
In short, I have no answers, but this might be a good thing to experiment with. I have several novellette/novella-sized works, and prefer to write in that length or longer, these days.