Flat Stone:The rocket bounces 4 times before exploding
Explosivo:Rockets have a chance to stick to enemies! If stuck,it will explode in 3 seconds.
Revelation:Rockets shoot 4 holy light beams in diagonal directions
Dryad's Blessing(Tainted Treasures):Has a chance to inflict "Germination" on enemies that survive the explosion.
Evangelism(Tainted Treasures):Enemies that survive the explosion will be struck by holy light beams!

i know there's already a brimstone synergy in the base game, but what about reworking it so that way you have a constant vertical brimstone beam that comes down from the sky and slowly follows the epic fetus targeting reticle?

Not going to make an entire doc of synergies, don't really feel like it, but I had an idea for Mom's Knife and Forgotten/Berserk! I thought I'd throw out
The crosshair spawns underneath the knife/bone when you throw them, and the missile will come down when the weapon is about to start returning to you
Kind of forces you to play high-range to not be exploded upon, but, it's an idea
If that's not a good idea, you could also just go with the same synergy as Spirit Sword, where the crosshair appears while you charge

Cain's Eye: Half of attacks deal double damage, but the other half deal half damage.

items idea:
flat stone: causes 2 additional marks to spawn with each shot, 1,5 and 1,25 distance to isaac then main mark. falling in order away from isaac at 1,5 time delay and 1,25 time delay.

tech 0.5: instead of firing at random when aiming the mark, tech 0.5 fires at the mark (tech 0.5 laser ends there) just before the rocket falling animation begins, dealing double the isaacs damage +5.

proptosis (ref: tear size): firing close deals higher damage and blast radius is larger, at "max distance" (calculated by range stat) the shot has atleast normal blast radius and damage

other idea:
fire rate: causes rockets to fall with smaller delay and higher frequency at 15 fire rate rockets fall continously 1 second after being fired
(the soy milk synergy looks too much like dr.fetus and eye of ocult)
multiple tear combos (inner eye, 20/20, mutant spider and so on) will in case of fire rate higher then 15 cause similar effect to montro's lung (based on how many extra tears isaac has)

range: increases or decreases fall delay but will not change firing frequency

tear size: increases or decreases blast radius based on tear size

shot speed (unchanged): increases or decreases the speed of mark

C-Section
Rockets leave behind an explosive Fetus Shot that can home in on remaining enemies, upon touching the floor, it explodes.

Astral Projection - Upon being damaged in either form, drop an Epic Fetus bomb
Paschal Candle - Fire an additional Epic Fetus Bomb for every stage of the Candle. The candle does not reset if you get damaged by your own bombs
Book of Virtues - Wisps mark enemies, where Epic Fetus Bombs fall a few seconds later
Stitches - Upon teleporting, drop an Epic Fetus Bomb where Stitches was once was
Quints - Fire an additional Epic Fetus bomb at each Quint Familiar's location
Suplex! - Before landing, create a ring of Epic Fetus Bombs around you
Decap Attack - Upon tossing your head, mark a Epic Fetus bomb directly at it

Nugget Bombs (FF) - When a friendly Dip dies, create a Epic Fetus Bomb marker
Pinhead (FF) - Upon an enemy having their sewn enemy die, create a Epic Fetus Bomb marker directly above them
Rubber Bullets (FF) - Upon landing, Epic Fetus Bombs bounce towards the nearest enemy before exploding
Black Moon (FF) - When a Cross vanishes, create a Epic Fetus Bomb that shares the same radius as the Cross
Telebombs (FF) - Upon teleporting with a normal bomb, create a Epic Fetus Bomb several seconds later, exploding and teleporting you back
Clutch's Curse (FF) - Upon being possessed, your next shot becomes a small ring of Epic Fetus Bombs
Time Itself (FF) - Epic Fetus Bomb Markers flicker randomly, stopping at random enemies.
Technology 666 (Reverie) - Epic Fetus Bombs create yellow Brimstones
The Infamies (Reverie) - Swap forms when killing enemies with Epic Fetus
Dark Ribbon (Reverie) - While in the damaging Aura, enemies are struck by Epic Fetus Bombs
Shanghai Doll (Reverie) - The Doll now wields a rocket launcher, firing an additional bomb where the Epic Fetus bomb lands
Jeweled Branch (Reverie) - Upon the Epic Fetus Bomb landing, create an additional 5 tears
Broken Amulet (Reverie) - The less luck you have, the more the chance the marker will become invisible
The Memory (Auri) - Mini-Auris now fire Epic Fetus Bombs. Does not hurt Auri
Seduce (Auri) - If an charmed enemy isn't permanently charmed, drop a Epic Fetus Bomb on them

Better my reflection idea:
Every time an epic fetus bomb drops, a very short fuse, spectral bomb is launched towards the player.

Eye of the Occult: Fires a Rocket in a Jar missile at the closest enemy
Tech 1/2/.5: Shoots a beam coming from the sky onto the crosshair, hurting any enemy under it
Scatter bombs: Cluster missiles
Almond milk: randomizes the firing interval.
Godhead: Missiles leave a damaging aura for a few seconds on impact

Ghost bombs: when the missile lands it has a chance to spawn a ghost missile that explodes on enemy contact (ghost explosion, so no self-damage), the explosion has a bigger radius than normal ghost explosions and is affected by Mr.Mega

Parasite: Two additional rockets fall after a .5 second delay after the initial one explodes, dealing 50% damage each

I don't know if you're still updating this mod but I'll be leaving these suggestions.

-Terra: Either when the bomb explodes it creates a shockwave like Dr. Fetus or it crates diagonal shockwaves. Honestly surprised how this still isn't in the mod yet. (Suggested via Youtube by peternight2517)

-Brimstone: Instead of the usual brimstone synergy, it can make the Forgotten Brimstone Ball where the rocket has landed.

Tech X: Insted af the usual synergy, it can create a Tech X ring on crosshair that always follows the crosshair. (Suggested via Youtube by unknownentityenthusiast6765)

-Spirit Sword: Instead of the mark shooting up a rocket, it can throw the sword to the mark spinning and send it back to you OR It can override completely the sword and use the Epic Fetus but when you shoot a rocket now it spawns spirit sword spin where the rocket landed.

-Fire Mind: It can spawn Crackles fire waves in diagonal directions when the rocket explodes.

-Rocket in a Jar: When you shoot a rocket with Rocket in a Jar it can spawn 3-4 marks on where the rocket has landed and spawn Epic Fetus bombs.

-Dr. Fetus: It should give you 2 tophats, 2 monacles and a higher explosion radius. I can help with the sprites if you want. (Suggested via Youtube by Slenderplier.)

Originally posted by LESBIAN CHAMPION SUPREME:

i know there's already a brimstone synergy in the base game, but what about reworking it so that way you have a constant vertical brimstone beam that comes down from the sky and slowly follows the epic fetus targeting reticle?

This is completely unrelated but you have one of the most godly usernames I've ever seen