These ideas may have been suggested before, or maybe they are a little unreasonable, or maybe they "ruin" the game. They are just some cool ideas I had and want to share. I am open to constructive criticism, but no hate please. I hope you read through the whole thing before you decide whether the ideas are bad.
So the main idea I had is this: You flip roles with the enemy (about half way in the campaign). So usually when you play this game, you go on a venture to save a Kingdom from some kind of "Dark Lord". The weird part is that when you reach the Enemy base, you build towers and defend against their troops still, doesn't make sense you know? In reality the enemy would be the ones building the towers and defending while YOU would be the one attacking...
The way this would work is as follows. Lets say in the beginning of the campaign you are the defender, like it should be. The enemy attacks you with soldiers, and you defend with towers. But at the end of the game it should be the opposite. SO, about halfway in the campaign (The middle 4-8 stages) it would be half & half. You AND the enemies can build & use towers to defend your half of the map, but you can also send soldiers down the path to attack and kill their hearts. Then when you get to the end of the campaign you wouldn't get any towers, just soldiers. The enemy towers would upgrade according to how much gold they get from killing your men. Upgrades would be the same, they would upgrade the respective soldier for each type (ie archer, soldier, mage, bombard). Maybe two separate upgrade pages. Instead of "Heroes" you get "Bosses". Maybe you can only use a boss on certain levels, or they cost gems to use, like a "Fat Boy" or a "Wrath of Elynie" it would cost 999 gems. They could also cost a lot of money, and you might need to ration money each wave so you can save enough for boss on the final wave.
Abilities you use when using "Attack Mode" would be like what bosses use on you in the game. If you are the attacker sending soldiers, your ability would be disabling towers momentarily. Or speeding your troops, or maybe just sending out a special unit.
So I know that this would change the the Tower Defense feeling completely. But honestly all three KR games are fantastic the way they are, but how long can they milk it before it gets stale? Maybe it change the game too far from the TD genre, but it still sounds really really fun. It would change how the game is played completely. Maybe it doesn't have to be implemented into the campaign at all. Maybe you can choose a gamemode. Or, some of the unlockable levels are played like this, when you get to send soldiers at enemy towers. Maybe there could be a "second campaign", or something. This could also work for multiplayer. One player manages enemies, the other manages towers. It could be Coop or PVP. Like lets say you can go to one of the Endless Stages, and you would choose what you want. There could be a little tab where you choose Single Player, Coop, or PVP. you would get the choice of playing with a friend (in which case you search their name and then you click on it and play with them), or you get matched with a random player. (I know multiplayer has been suggested a million times and I bet they were all similar to this, but I am just trying to get my ideas out there)
I suppose these are slightly unreasonable ideas, but this could be simplified down. Mostly what I would want to see most is that you get to "switch roles" with the enemy. Being able to try to break through enemy defenses sounds super fun. Having bosses sounds fun as well, rather than heroes.