Some sort of cutscene system, where cutscenes can be triggered by many different events such as talking to an NPC, walking over a square, and other things which I can totally think of. During a cutscene there should be lots of different options you can have, like dialogue, moving the players about, giving a player an item, and plot dependent actions. You could use some sort of scripting language like XML or maybe Lua. It would also be nice to save and load levels as XML, then you could have a level editor which generates a file which you can share with others. But I suggest you encrypt the XML file first then decrypt it when you come to use it, so it just looks like binary code to most people.

I am going to add more ideas that I have in separate paragraphs. You could encrypt the XML files with a password, if the user so desires, so that one can only play the game with the password. Or you could have an option to just use a default password, so anyone could play it.