You can change virtually everything in the game. You can select things from the library and make an adventure. Or you can use the random generator. But if you want to create a new Creature or Item, you have to do a little bit of programming. However, RuneSword is NOT a modified C or Java language.
All programming is done by selecting properties and picking from lists.
It would be great if users submitted a lot of tomes for others to play. Ask any pen and paper dungeon master about how much preparation is necessary for an original adventure. That's why the random generator is great, to help those who don't want to do everything from scratch. That would be cool, like an ongoing dungeon master library. That's the idea.