Now I know KR2 is in 'very early Alpha' and something like this would take monumental effort, but if anything has proven to extend the lifespan of a single product it would be these in this order :
1. Multiplayer
2. Competitive play
3. Moddability
4. DLC
What we'll cover in this topic is points 1,2 and maybe 4.
That up there is all I'm saying to try and convince you guys this is a good idea, from here on we'll talk about the usual ways in which TD does player vs player
I believe the original method was conceived by Line Tower Wars, which was originally a Warcraft 3 custom map. You got periodic income which you would spend on towering up or sending hordes of enemies at your opponent (you buy enemies from a menu), and sending enemies contributed to making your income bigger, so it boiled down to balancing your defensive and offensive expenditure so that your enemy runs out of lives before you do.
There is another method that would be unique to KR right now because of the barracks system, do away with the creep-menu at least partially, and let your own spawning structures play the role of pushing into the enemy base.
What this means is that your soldiers will not be tied to a rally point as before, but rather keep moving forwards as long as nothing blocks their way. Of course, some balance adjustments would need to be made so that your soldiers don't drop like flies before the might of their towers, but you get the idea.
There is also the fact that you can quite easily create simple benefits that people would be more inclined to buy in a competitive game.
So what do ya say?