Lil' Frost Rider(Revelations):
Upgrade 1:The Ice Cube(with or without Charmed Pooters in them) will shoot 6 projectiles in random directions when destroyed. Ice Coubes leave creep when they slide also!
Upgrade 2:Projectiles from the Shattered Ice Cubes can cause slow effect.
I'm curious to see if you can do upgrades to familiars from certain mods like revelations.

I can upgrade modded familiar, I did it for Marshmallow already !
I don't play enough with Revelation, so I don't really know every familiars. Don't know if I will upgrade them, but thanks for the comment ;)

Lil' Monstro:
Upgrade 1:When Isaac takes damage,the familiar will do the super jump attack on the enemy that hurt Isaac.
Upgrade 2:Its super jump attack makes him shoot projectiles in random directions
I thought these upgrades could make the familiar fit like the Monstro Boss

There is already an upgrade for this familiar, which I think is good enough. Thanks for sharing this idea @cadetpirx, but will keep the existing one :)

Now that the mod works, may I suggest an upgrade for bumbo.
Upgrade 1: Evolves faster. It may drop a random trinket once he reaches his second stage. When it reaches it's third stage it will drop either 3 bombs, 2 keys, or one card/pill. On it's final stage he'll keep collecting coins, but he'll exchange it for pickups like a bum friend.

Upgrade 2: Coins it grabs are shared between you an bumbo. So if he grabs a nickle you'll recieve half of that payment, so 3 coins. On his third stage he'll mimic your tear effects. His final stage he will ocassionally shot out 4 tears at the closest enemy to you.

Thanks for sharing those ideas, I may consider some of them. Bumbo is a complicated familiar because it has 4 states, but I like the coin sharing thing !

here some ideas :)

rune bag: higher chance of spawning runes/has a small chance of spawning 2 runes unstead of one.
sacks of sacks: higher chance of spawning a sack/33% chance of spawning 2 sacks

dark bum: harmful spiders replaced by half soul hearts unstead, also slightly increases chance of black heart/has a tiny chance of spawning an item from the demon beggar pool.
bum friend: will stop taking coins if you have 25 coins or less/has a tiny chance to spawn an item from the beggar pool.
key bum: no longer takes keys if you have 10 keys or less/tiny chance of spawning an item from the key beggar pool or the paper clip.

gemini: damage up and can detach itself after some time in the room (resets each room)/ damage up, speed up, is always in detached state.

multidimensional baby: will follow's isaac's movements sooner/tears that passes through the familliar gains a 1.3x dmg multiplier

succubus: increased radius/isaac's dmg inside familiar radius are doubled.


revelations:

birth control synergy:
this is a very complicated one and i will try to explain at much as i can:

consuming a super familiar will add an aditional +0.2 dmg
consuming an Ultra familiar will add an additional +0.3 dmg on top of the +0.2. (that makes +0.7 dmg in total)
you can still upgrade familiars when they're consumed to increase isaac's dmg.
placing a familiar on the sewing machine will remove the items and the dmg they gives upon consuming, until you pick it back or upgrade it. picking up a familiar on the sewing machine can't drops any pickups (especialy the ones that drops them upon consuming like the bag items)

Thank you @jgmc2 for sharing all those ideas !
I will consider those Bum ideas and bag ideas. I already think about something similar for Gemini but it seems hard to code.
About the Revelation item "birth control", I have to test it a bit as I do not know it. But I'm not sure if I'll make a synergy, seems complicated

in stead of not blowing up by the bomb it should blow up by the bomb wen you get your buddy back

Anthology 2 adds two familiars,I thought up some good upgrades
Lil' Bony:
1.Chance to shoot a black bone which explodes on impact,and the Bones it shoots now scale with Isaac's damage.
2.Shoots bones in 8 directions when Isaac takes damage,similar to Brittle Bones when losing a bone heart. This attack is effected by the previous upgrade
Living Broomstick:
1.When it cleans a creep,it will shoot a tear based on that type of creep it cleaned up.
Black:Inflicts Slow effect
Green:Inflicts Poison effect
Yellow:Deals 1.5x damage
Red:Inflicts Bleeding effect
Blue:Normal Tear
Familiar's projectile damage scales with Isaac's
2.Projectile Damage Up!

more ideas to unmentionned items:

isaac's heart: shoot's tears in the 4 cardinal directions if an enemy is near (7 tear rate), follows Issac faster/will try to avoid tears and contact, enemies are more likely to target Isaac unstead of the heart, projectile tears up and dmg up. (a very bad item needs a huge upgrade)

dead bird: dmg up/is now always activated

Incubus: deals contact dmg and can block shots/will always be the closest familiar to Isaac in the familiar line, contact dmg up.

Holy water: puddle dmg up, puddle size up/puddle size up, can be used twice in a room

Milk: extra +0.2 speed when being hit/puddle size up and +1 extra dmg when being hit.

pointy rib: dmg up, applies bleed effect/dmg up range up

lil chubby: dmg up/will stick to enemies for 0.5 seconds before continuing his path
BIG chubby: dmg up/will try to home into enemies near him hile continuing in the same direction

Acid baby: can sometimes leave a puddle of dmging creep/no longer spawn bad pills, more chances of good pills.

yo listen: moves faster/will taunt enemies (they will try to attack the familliar unstead of Isaac)

Hallowed ground: now has a small aura that gives -1 tear delay/bigger aura, the aura also gives +0.1 shot speed

finger: dmg up, invisible laser is now visible/ will automatically point at the nearest enemy

Lil chest: can drop 2 pickups unstead of 1 (50%)/has a tiny chance of dropping Items from the item room pool.

i would like to share some retribution items but... the devs don't want support with this mod yet.

You guys share good ideas, I will consider them for the future ! :)

Ideas for familiars from Baby's First Shapes:
The Square:
Description:Deals 5.75 damage to every enemy per second while in a current room that is not in the shape of a square.
Super Form:Damage up,Pushes back enemies every time it flashes.
Ultra Form:Now attack enemies in rooms that are square shaped + Damage up.
The Pentagon:
Description:When on an enemy it sends an airstrike missile on it
Super Form:Missile explosion pushes back enemies around it,the marker will stay on the enemy that the familiar touched.
Ultra Form:Its missiles now affect bomb modifiers

- Holy Water -
1. Enemies killed by creep have a chance to drop soul hearts.
2. Unleashes a "Crack the Sky" effect when activated.

- Ball of Bandages; Buffs completed Bandage Girls -
- Bandage Girl (Completed) -
1. Attracts enemies like Punching Bag.
2. Enemies have a chance to become friendly when they would be killed.

- Cube of Meat; Buffs completed Meat Boys -
- Meat Boy (Completed) -
1. Leaves puddles of red creep on the ground.
2. Occasionally spawns yellow locusts while there are enemies around.

- Dead Bird -
(Reference to a comic where Ed first showed it, and it promised to grant "evil wishes".)
1. Enemies killed may drop black hearts.
2. Chance to spawn black hearts, red chests, evil trinkets, and devil items on clearing a room, based upon damage taken. More damage = higher chances / better rewards.

- Smart Fly -
1. Will actively attempt to intercept projectiles.
2. Upon killing a fly enemy, converts it into another Smart Fly for the room.

- Charged Baby -
1. Slightly more likely to freeze enemies or charge Isaac's active item. Sometimes fires random lasers.
2. More frequent lasers, and occasionally fires a Sulfuric Acid tear. Can charge Isaac's active item an additional time per room.

- Rune Bag -
1. Has a 25% chance to spawn a new rune whenever a rune is used (not counting active items that duplicate runes).
2. Replaces previous effect with a 40% chance to spawn the same rune when a rune is used, and an additional 30% chance to spawn a random rune if the same rune isn't spawned.

- Incubus -
1. Shots from Incubus have a chance to cause confusion.
2. Defeated enemies have a high chance to drop half soul hearts.

- Succubus -
1. Shots fired through Succubus gain pink damaging aura.
2. Succubus gains a smaller magnetic aura that pulls in enemies and bullets.

A suggestion for the "Bombby" mod: https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1087257565

- Bombby -
1. Faster recovery after exploding. Less likely to drop bombs near the player.
2. Bombby no longer hurts the player when he explodes (but can still heal them with Pyromaniac).