well.. maybe you can combine the story line of book 1 to book 3 ..
it uses the latest game engine..
the map of book 1 and 2 can be explored on book 3
just a thought
or maybe adding a feature like.. go straigth to book 3's storyline for people who already played book 1 and 2
of course if you start from book 1, mobs on book 2 have x2 damage or 4x.. haha =3 *meow*