An additional baby slot would be nice, but maybe if you are against just adding one as base capacity, then how about adding it through Tainted Isaac's Birthright?
A suggestion for Mongo baby, instead of just mimicking the fire style of the baby behind it, maybe also the tear effect? and also increasing the heart cost from 125 to 150 if the tear effect part is added.
King baby could be used like the Torchwood, but instead of setting tears on fire, It adds a 10% damage bonus and one time tear pierce to babies around it (3x3). It can also shoot normal tears, but it is not affected by it's own effect. It would cost 200 hearts. Multiple King babies would not increase the effect.
How about you make a branching path, this will probably be a coding nightmare, but imagine having a choice at which path you want to go to (like the alt path in TBoI). A normal route where the rewards are normal and the difficulty is the same, and an alt route where the reward is higher, but the difficulty is increased (increased meaning more challenging enemies and not bullshit combos like a bunch of worms). While I am at it, why not decrease the worm's movement speed by 10% and attack speed by 5% from the current speed. They are just to fast and unavoidable if you are caught with a recharging cube of meat.
I also think that "Hard mode" should be taken off the board for now. Try to finish the base game before trying to make an extra difficulty.