Just wanted to let you know that I'm trying to define a new type of mission at the moment. I was wondering if someone tried this by now, and what difficulties were experienced while doing so.

Are you "cracking" the source code? Because I see no way of "adding missions" without doing so. Adding/changing data in files to get a new galaxy configuration or change unit stats (or add characters as you have stated) is one thing, but this ... ? I wish you the best of luck