I was thinking about what the real problems were with TD and it's this: the creeps don't fight back. If you had the ability to dynamically place turrets wherever to make mazes, and also be able to block creeps in, then it would feel much more dynamic and fun.
Imagine the following scenario: You're playing Hypothetical Tower Defense game, and you slap down some cannon towers (or equivalent) in a pattern to make a long maze. The creeps will march the maze (as they refuse to stop if they can make it through otherwise), take fire, and go along. Let's also say you put down a tower that stopped up the maze (so the creeps wouldn't get through). They would realize that, and head for the towers on the shortest path to the goal and bash them down before continuing. You then have to reconfigure your defense line to stop this sudden creep hemorrhage and possibly re-tweak your design so it doesn't get "plugged" again.
Or maybe you do this instead: you design your defenses more like a line of tower "trenches" that creeps have to bash through, with weak spots and traps on them to get those that make their way through.
The game balance would have to have the following properties:
* individual or waves of creeps can only slightly damage towers while on their way to the destination
* When the maze is plugged, creeps can focus significant firepower on getting out (to prevent shooting fish in a barrel situations)
* Healing turrets should be expensive (whether through expensive charges for direct repairs, or weak for their cost healing turrets)
Unique towers could include:
* Barricade towers (which have more armor but no weapons)
* More types of trap-like towers
What do you think? I'm typing this up as sort of an AVWW applicable idea, which i wanted to get to X before he got major mechanics in place. I don't expect any major "this will/won't go in" (he hasn't even started development), but i just thought it would be cool to do a little watercooler brainstorming for TDs.