I always have ideas for another book. Some of them are as follows (now don't be stealing these all you authors out there):
Missions: Changing the way the church does Mission work
Christian=Disciples: If you are saved, does that make you a disciple or is it separate? If only disciples were believers, how would that revolutionize the Christian world?
A fiction book on the steps a missionary takes in arriving on the field.
Balance in the Christian life: There are so many books out there on secrets to the best prayer life, devotional life, going radical for Christ, but what about learning to just maintain a balance rather than getting burnt out.