The game would be turn based. Each player would start out with a hand of cards, or tokens. Each of these tokens can be anything from, units to abilities or boosts, debuffs and the sorts. Some of these will be energy producers, and generate a small amount of energy to use each turn. Which doesn't carry over each turn and instead resets. Energy is used to play tokens.
The play area would also be a grid, probably 7-9 units tall. Allowing each player to place tokens 2-4 units deep on their own side. Each turn, if a token can move it moves the stated amount. Tokens can never land on another token, If to opposing tokens would occupy the same tile, the moving token would attack. If your unit reaches the opponents side, it deals damage to the player. The goal being to reduce the players health to 0.
I don't know if that counts as a TD. A TD includes towers and enemies in a lane going towards a point -- this would be more or a Tower Wars. Instead of turn based, maybe real-time? With creep cards that could travel one of the lanes/a single lane and fight oncoming monsters, with tower cards that can be placed beside the lane(s) and fire at incoming enemies. Buffs could include stuff like faster card drawing (as in, twice/trice per "tick"), making the enemy throw away one of his cards etc.
In any case, do explore the idea, it seems interesting
Then maybe more of a RTS-TD. I just like the idea of useing abilities and other more strategic pieces, instead of just plopping down towers and hoping for the best. Let's explore the idea of real time combat. Though that would throw out the idea of drawing from a stack of cards, I think you'd either have to have a huge stack or run out eventually.
So say the player can choose a group of tokens to use, say 10 - 15 tokens, these would randomly popup as available. The player would have a 5-7 token tray of currently available tokens. Every time one is used another takes it's spot. Choosing randomly from the token types the player selected. When a player plays a unit(creature) token it immediatly begins to move along the column it was placed in. If in range it fires upon oncoming units. If it reaches the end it damages the player it hit.
The other part of that. I wanted to make long range units seem important, so what if they were able to attack units in adjacent columns? Instead of being able to jump over units. Then abilities or boosts as I was thinking of. Would be things like, +10% attack power. Also things like "Deal 50 damage to target unit", "Gain 5 life". Could help your units do more, damage or kill enemy units to allow yours to get through.
Debuffs could be things like, "Slowed for 10 seconds", "-15% attack power".
As a theme I'm thinking more a futuristic look, tanks, helicopters mass amounts of robots and lasers.