You're not locked into anything yet. You can do pretty much whatever you want. You can display randomized messages if you want, but you need to figure out exactly what all the features are so you can plan the code and XML format. Certainly the code could have a random function, and if you want, the XML file could have an indicator in it to say when to pick a random message. But remember that if you want complicated features, you might have to write complicated code.
