Here is a link to the bukkit documentation for projectiles. Use this to choose a different type of entity to use as a projectile (look at the list under "All Known Subinterfaces:").
I made a fireball with a giant explosion!
Make at least two different types of projectiles work at the same time. Add as many as you want!
HINT: For my version I used "Material.STICK" to shoot eggs! You can browse other materials here.