Suggestions would be:
Make the cannon modes mean something
Add in a mini wizard mode if you've played all three multiballs
Add in a mini wizard mode if you've beat all three multi balls
Make the horned inserts mean something more than they currently do
Make the lower playfield activate with the upper on some multiballs if you stack correctly
Would be nice if there was some sort of two ball multiball that you could earn for stacking with other things (like gollum from LOTR, I&S on TSPP, Doc Ock on SM, etc)
Add the OPTION for some sort of ball saver off a made cannon shot (NOT BY DEFAULT, some people are sensitive to this, lol, on my game the ball drains off a make like 30% of the time, so frustrating)
Combo multiball for 50 combos