It feels like tower defense is largely the same since the late 2000's. Sure, it was quite popular back in the Plants Vs Zombies or Bloons days, but it's not quite mainstream anymore. When was the last time you saw a TD on the front page of Steam or Itch? It seems like Bloons is your only option if you want to play a TD these days. So how could you break through a seemingly monopolized genre?

Playing Alone Is Archaic

To compete with a game like BTD6, you have to be at least on par with them. Because of this, you need to have a multiplayer feature. A TD is fun, but it can start to get boring after a while, so it's almost necessary to add a way to refresh it by playing with friends.
Just set up a little LAN connection and suddenly the game is a lot better.

I Want My Own Maps

Let's say you've made the game, and you've made 10 maps, what does someone do once they've played them all? It's all the same after that, it's boring. Giving players the ability to make their own maps gives them infinite maps to play. If you've made a program that you make your maps in, just bundle it in with the game!

Simple, Yet Complex

A conehead appears, you can probably guess that it has more health. Very simple design, and easily explains the entire premise of the zombie. This doesn't mean you can't have complex combinations. In BTD6, some balloons have camouflage, so only certain monkeys can see them. Seems simple, but you can mix it with lead balloons that can only be popped by certain projectiles. It takes a few simple rules to create complex behavior, not only in enemies, but also in the towers. A tower that slows enemies down, poison them, and so on. Now a lot of these ideas have been done to death in TD games, but with how simple these premises are, it wouldn't be too hard coming up with a few new fresh ideas.

On Its Head

To make a riskier move, there are a lot of ideas to radicalize a TD game. You could have a first-person 3D TD, or the player is the enemies, or you control a character rather than towers. There's a lot that you could do. But one that I think would be interesting is making a PVZ style TD, where there's no track, it's just rows. This style of TD is probably easier to make, but it better have something that makes it interesting, otherwise I'm just playing PVZ.

What makes your TD game stand out? Why would anyone pick your game over BTD6 or PVZ? The TD genre is feeling a little dry, and I don't think it'll be long until someone has a great idea for a fun game.