I saw that a Steam friend of mine was playing Gary's mod and I asked her what she was up too. She said that she was playing a TF2 tower defense game where the classes were the towers.
I've never really liked Tower Defenses that much to be honest with you. They tend to bore me with the need for a long lead up to action they have to have in order for them to not become too hard too fast. I find it takes too long to get any noticable feedback about whether what you are building is working or not and then you rarely have the resources to alter what you are doing to improve the situation that much. It doesn't feel like a game to me, it feels like running a simulation and seeing if how you set it up was correct or not only you have no fast forward key.
But that said, I realize that it is a very successful genre and I believe it is for the same sort of reason that a game like DotA is; it takes something that is normally only tackled by the RTS genre, big battles, and simplifies what you have to do in them until it's less stressful and therefore more fun. DotA does this by simply making you a very powerful unit who fights along side an AI controlled army, a demigod if you will or a living legend from a league of other such legends. Tower defense accomplishes the same sort of simplification by changing the scenario from a battle to simply base design and defense with no units that need controlling, just placement.
But anyway, I think I thought up a decent idea to make the genre more palatable to me; JERRY RIGGING!
Remember on any Star Trek show when they got into some big battle or some overly polarized gravametric field was disrupting the warp core and preventing the ship from creating a stable warp shell? When shit hit the fan, what did they do? Well, Geordi or Miles O'Brien would re-route power from something. Deep Space Nine was great for this sort of thing because the base station was all Cardasian and so anytime they had to repair or upgrade some old system it was almost always somewhat Jerry-rigged to work with the Starfleet components and tools they had to work with. Alright, so maybe this always resulted in someone openning a panel somewhere and pushing some simple instrument up against something while it made a funny noise and perhaps waggling it around a little, and maybe people never got dirty or had to lug big components anywhere, but the idea was cool.
tldr;
The Connected Systems Tower Defense Game
I'd like to make a tower defense where all the towers can be connected somehow and where the player can re-route power around to trouble spots as they arise. The idea is that every tower you make does two things, it shoots or attacks in someway at mobs that come down the pipe, and creates power for a certain kind of system or network.
For instance, a lava funace fires molten lava balls at the enemy but it also creates heat that can be shared with any other heat system based tower that it is connected to. Similarly a Tidal Fountain fires out waves all around it and creates pressure for the hydraulic system, thundering capacitors shock anything near them and generate electricity for the electric grid, hurricane fans make air pressure for the pnematic system, etc.
Some buildings might just make power for one or more of the systems and not have an attack, and some might just drain power to perform very powerful attacks but give nothing back to their systems. Also, there could be special buildings that allow you to transfer power from one system to another. A water wheel could convert hydralic pressure into electric power or a steam generator could convert equal amounts of hydraulic and heat energy into electric power, a heating element turns electric power into heat that sort of thing.
You could also maybe turn natural things on the map into energy of different types depending on what sort of collectors you have or even have recycler buildings that can catch the extra damage run off of AoE towers and put some of it back into your systems in the form of energy.