Update

I'm in the middle of working on internal changes to the game to support some of the new features. The following are all the changes I have planned, both internal and external:

Remove rush delay lockout (done)
Add variable sized levels (done)
Move level data to files (done)
Change incoming display to be dynamically drawn instead of at level load time (done)
Change enemy data to be dynamically created instead of at level load time (done)
Add hard mode (done)
Remove scoring for building towers (done)
Increase scoring for rushing (done)
Change targeting to closest to exit instead of oldest on screen. (done)
Improve background graphics (10% done)
Add endless mode (50%)

Add tower targeting modes, including option for "classic" and "new defaults" (delayed?)
Add flying enemies (delayed?)
Change towers to support flying enemies (delayed?)
Add rushing button, with a "rush at next opportunity". (canceled)

Fix bug on greenskin gangs level
Fix open build spot in water on level ?
Fix tiny dragon head graphical bug

Did I miss anything?

I'll edit this post as I check things off.