I'm gonna start working on the free challenge story this week, and it should be ready next Sunday for download. I will also be making updates to the world editor, making it prettier and more user-friendly.
As for the main story and game engine, I'll wait for more bug reports and general suggestions before releasing another update. I was thinking about adding mini-health bars to all the teammate portraits and putting some more details into the maps. Also, I want to correct the corpse angles so dead people don't stick through walls when they are on the ground. Otherwise I think the main story and game engine are done.
The long-term goal is to make a paid DLC for maybe $2.99. It will be a totally new story, about half as long as the main story. I already have a bunch of new assets and ideas for weapons, bosses, etc., but I don't know if it should take place in Panama or Cuba. I am still open to suggestions before I start really planning.