Monster Regeneration in exchange for Rebirth Points.

A square on the path.  It restores some of the Health of any injured monster as it goes over it, and has a chance of returning a teeny amount of Rebirth Points as it does so.

Called a Healing Station or Hospital or Clinic.

Could work like the Spikes and have one upgrade option for healing Health damage and another upgrade option for healing Health as a %.

It would not be very efficient for building Rebirth points, but would add to the gameplay options.

Narrow path: no overtaking.

A square on the path that makes the path appear narrower.  Monsters may not overtake one another on that square.

The main purpose of this is to make sure a Rockster is in front of any Ballys so it takes the fire from Gunshots and Laserguns so it is not wasted on the Ballys.  It also slows the Ballys when that happens so they are vulnerable to any Spikes on the previous square or any Guinshots or Laserguns behind them.

It might be it is not effective 100% of the time and so has 'Effectiveness' as an upgrade %age.

Style or 'Kissing Gate' or Turnstile.  A square on the path that can only contain one monster.  If there is a monster already on that square, monsters start stacking up behind it until it has passed over.

A Rockster will hold up all the Ballys behind it.  Rocksters that have appeared from Instant Spawn Chance will get spread out.

This will be like a very effective 'Stun' or 'Sleep' function, so will need to be very expensive.

Could you add Mobile compatibility? I wanna play this, it looks fun.

nvrmind. anyways, what about a new enemy? got a better idea: Nitrogen. it slows down your turrets and buildings.
Rocket launcher: shoots homin projectiles, and is affected less by Nitrogen.
enemy: Ice. could be easy to kill, but VERY fast. doesn't give high rewards due to its low health.

Turret ideas

 freeze turret, flame turret, slam turret to deal area dmg, stun turret.

Monster ideas

Stun monster to stun turrets. Initial 5-sec cooldown upgrade for more coins. initially worth 30 coins.

Boss Monster for a lot of money, but takes 15-sec spawn time. 2000 hp, and can also stun. stun cooldown 15 secs. initial speed is 0.50. Initially worth 5000 coins.

Bolt Monster. extremely fast but has low hp. initially worth 15 coins.

Healer monster heals other monsters in a radius by initially 25% of hp with 10 sec cooldown. for boss, heals 5 hp. upgradable. Initially worth 50 coins.

Rebirth ideas

You should add a part on the turret rebirth upgrades that you can upgrade their base stats in. Also, you should add upgrades that let us exceed the cap value.

Using some of the other comments I've seen with some of my own ideas.....

With xxxxxxx RP you can "Re**something**" in which the game "soft resets" and you have to re-unlock all the structures. Each tower gets a new buff or something that is upgraded with RP. Including some form of a 100% chance to instant spawn mix in a random tile placement restrictions so that you can't simply rinse and repeat the exact same layout every time. Not sure what's possible but another addition would be to have turrets aim at target that you click to add some "control".

Maybe adding little drones that specialize in single target damage might be pretty cool

(sorry about my bug report I made earlier)

Add modes, I would also recommend adding a title screen in case the player wants to change modes. Here are some things I have: 

                                                                                                                                 WELCOME TO DUALFORCE IDLE, DO YOU WANT TO:

                                                                                                                                                                        Start new game

                                                                                                                                                                               Continue

                                                                                                                                                                           Change mode

Modes: Sandbox: Have infinite money, Spawn monsters, and Infinite rebirth points.  Master Mode: Items are much more expensive, monster upgrades are cheaper but gives monsters more strength than usual, monsters give less rewards.