I won't add damage values, health, cooldowns, or costs, because I'm kinda terrible at balance, but here's some ideas:
Lil' Abaddon: Charges up, click on it when fully charged (like Coconut Cannon or similar) to deal high damage to everything in a 3x3 radius. Ignites Paschal candles in dark-red flame, making them fire tears slowly.
Reaper Baby: Somewhat defensive, deals damage to all creatures in up to two tiles in front (similar to Bonk Choy, referencing Samael).
Infestation: When killed, the monster that killed it is infested with flies. When that monster dies, many flies similar to that of Parasitoid's fly out of it.
Halo of Flies: Placed on top of an existing baby, somewhat defensive (Pumpkin is underrated)
Lil' Spewer: Slow firing, damages all grounded enemies in front of it. Can be given pills to have different firing patterns. Fires faster when placed next to Acid Baby.
Troll Baby: Instantly explodes, dealing massive damage in a 5x5 area. Makes a crater below where it was placed, making it impossible to place anything in that space for the rest of the stage (Doom Shroom).
Ball of Bandages: Similar to Cube of Meat, but with less health. Reflects projectiles that would damage it.
Worm Friend: Instantly kills the first enemy to touch it, disappearing in the process (Tangle Kelp).
Horn Baby: Initially fires quickly, but slows down over time. Dies after a certain amount of time. (referencing Chronos)
Salt Baby: Charges over time. When at full charge, click it and then click an unoccupied square. Salt Baby will jump to the square, dealing large damage to all monsters in the square, then jump back. (Edith)