Bomb: Moves slowly, has a short fuse, if you don't destroy it in 6 seconds, it will explode, releasing 4 Black Bloons in each diagonal. If any get caught on another portion of the map, it will remain there. If they don't land on the track, they will act like Bloons caught in the Druid Whirlwind and go to the closest track tile. If you manage to destroy it, it will explode visually, but not release any children.

This encourages strategic play, deciding if you want to earn extra cash by popping the Black Bloons, or taking the safe route by denying the Bloons from ever existing.

Children: 4 Black Bloons, 2 Health

Crystallised: Moves slowly, projects a shield of radius 2 and Health 5 that will block any projectiles from hitting the Bloons within. Regenerates the shield in 4 seconds. Starts with shield.

This encourages micro, since you want to pop the Crystallised Bloons ASAP. It also counters High Damage but Low Output towers like Snipers and minimizes the effect of area damage.

Children: 2 White Bloons, 3 Health

Shooting Star: Medium speed, lands anywhere on the map from 25%-50% completion rate.

This encourages spreading your defense out, instead of having one huge cluster of death somewhere. It also encourages playing longer range towers and full map rangers like Snipers and Monkey Sub.

Children: 1 Yellow, 1 Health

EMP: When destroyed, releases an EMP of radius 4 that temporarily disables robotic towers (Tack Shooter, Submarine, Airplane, Heli, 030 Super Monkey, Banana Central, Spike Factory) for 3 seconds.

This encourages the use of non-robotic towers and the strategic popping of Bloons, as to pop these before they reach any Robotic Bloons.

Children: None, 1 Health

Here are some mockups of the Bloons I made:

Bomb, Crystallised, Shooting Star, EMP