So here is what I think is the core challenge: To preserve the familiarity of tower defense games but to introduce new mechanics to challenge the player.
My first thought was to change the creeps or the towers themselves, but after a bit of time I abandoned this thought experiment as it's not in line with the first half of my goal (to preserve the familiar). Therefore, we need the six basic towers I see in every game:
- The cheap, medium-damage and fire rate tower (starter tower)
- The fast but weak tower (machine gun)
- The slow, long range tower (usually a sniper or mortar; optional splash damage)
- The tower that slows down the bad guys
- The tower that poisons or burns the bad guys (Damage over time)
- The tower that does splash damage
Some games change up the creeps - the Protector series does an excellent job of this, with various creeps immune to/made stronger by/vulnerable to various types of damage. I think this is a good step towards our goal.
And then my mind turned towards the economy of TD games: Kill an enemy, get money/credits/energy. This is the only thing I have never seen changed.
Well, let's do something about that.
I envision a game where you play the role of an independent contractor charged with the static defense of a space colony. You have numerous contracts with various weapons companies, all of which specialize in turret defense. As one of their top customers, and due to the media attention your company is receiving, your successes and failures have a direct impact on the stock market value of these turret defense companies. Therefore, your primary means of income is not killing enemies but trading stock.
Killing an enemy with a turret made by Altair Industries, which specializes in ballistic weapons (the Rifle, the Machine Gun and Shotgun Turrets) drives their stock value up, while having a negative effect on Betelgeuse LLC (specializing in self-propelled explosives such as missile strikes, mortars, and rocket mines) as the two are in direct competition. Driving a stock price too low may result in BLLC sending mercenaries to the colony to discredit Altair Industries technology.
Other companies include CanDo Construction, whose upgrades allow you to alter the path the creeps use (through barricades, carving new roadways, moats, and other innovations), and Delta Management Solutions (whose "turrets" actually just boost the functions of other nearby turrets - a Media Team increases the stock value of successful kills, especially bosses, while Motivational Speakers might lessen the effects of failure and drive the fire rate up).
Pausing the game allows you to engage in stock trading, while randomized global events keep you on your toes - like the real market, a canny investory can cope with almost any events. Special cards are collected by completing side missions and destroying bosses, and these trump cards allow you to turn events to your favor.
Because every game now needs to have friggin' multiplayer, 4 person matches allow a dynamic market while defending separate maps. An additional sandbox mode allows players to blast away at creeps with your investors supplying money for you but only in between waves - so the resource management aspect stays at the forefront.
By combining economic strategy with tower defense, I hope to create an innovative, engaging experience without branching too far away from what the modern games is familiar with.