On the subject of characters, one thing I've noticed in some fan films is that they add tons of characters. This makes things harder for everyone. It's hard for the audience to keep track of everyone. It's hard for the writers to come up with things for everyone to do in a story. It's hard to find costumes for everyone. But perhaps worst of all, all the characters wind up boring.
It would be far better to have just a few characters that are interesting than a lot of boring ones that don't fill any role.
The only fan film I've seen to handle large casts well is Intrepid. By giving most characters some brief moment of action, or a funny line, they become memorable. When someone's biggest role in an episode is to say "Shields at 47%!" you forget about them.