Here's my idea :

SCP-049 : The Plague Doctor. This little item is obviously based on the Euclid SCP who "cures" Humans by killing and changing them into zombies for him. As a function, let's say Isaac will be able to send syringes who can bring a "Cured?" effect, similar to poison, but extremely violent and really short. If the enemy died due of that, it has a chance to become an ally. Isaac will have some negative effect though and might hallucinate ... As the Item Pool, The Shop or the Devil Deal pool seems legit ?
(If you want to bring foes based on SCPs as well, he can be interesting as well ! Though, I've any ideas ...)

@Galacta K. SCP-049 might be added in the future as an enemy that infects other enemies, turning into zombies upon death, or something like that. SCP entities are most likely to be enemies instead of items, like SCP-106 and SCP-173.

Yea, I expected that ... I know a little SCP looking like a spider. I'll do the suggestion later though, cause I forgot its number ^^"
EDIT : it's SCP-632 !

Cool! I even have some sprite I made a long time ago if you're interested. Here are some of my ideas for some items:

SCP-1123 - Atrocity Skull
12 charge active item that when used gives the amnesia effect and randomizes stats.

SCP-012 - A Bad Composition
Active item with no charge. When used it damages you, creates a large red creep pool where you stand (similar to Lemon Party) and gives you a red creep trail effect until you leave the room.

SCP-018 - Super Ball
One (?) charge active item. The ball is thrown like Bob's Head but the projectile is like Rubber Cement and persists indefinitely. Every time it ricochets it increases in speed until it reaches a maximum limit. The faster it gets, the more damage it deals, eventually getting strong enough to break rocks. The item can't be used if a ball projectile still exists in the room. Maybe for balance reasons it may leave the room when it gets ridiculously fast by making a hole in the wall.

SCP-447 - Ball of Green Slime
Passive that covers Isaac's eyes with the green slime, upgrading his tears. Maybe it gives bouncy tears, increased dmg, tear speed and range.

SCP-458 - The Never-Ending Pizza Box
Similar to Yum Heart but has two charges and heals half a heart, making it more manageable than the Yum Heart. Isaac gets fat after using it too much (visual only?).

SCP-776-ARC - A Slightly Unlucky Die
3 or 6 charge active item that when used does a random effect out of 6. These are:
*1 - zaps a random number of enemies with a blue tech laser and gives you the Jacob's Ladder effect for the room;
*2 - freezes and slows all enemies in the room;
*3 - floods the room with water (like the Flush), cosmetic effect only;
*4 - breaks all or a random number of rocks, slightly damages enemies and gives them the confused status effect;
*5 - shoot 5 explosive Fire Mind tears around Isaac;
*6 - not sure for this, maybe a windstorm that pushes enemies randomly around the room?

SCP-999 - The Tickle Monster
A familiar that charms enemies on contact. Uses clot as an animation base (I have sprites for that).

SCP-1025 - Encyclopedia of Diseases
2 charge active item that when used gives the passive effect of a random disease/injury related item for the duration of the room. Examples include IPECAC, Compound Fracture, Common Cold, Anemia, Leprosy and others.

If you'll do enemies I imagine SCP-049 as a boss that chases you and instantly kills you upon contact damage (unless you are undead already, like Blue Baby and The Forgotten). The boss room will have some doors that occasionally open as a group of zombies walk in the room, just like Ultra Greed does but obviously with not that many enemies.

SCP-999 is already in the list of which may come in the next pack, if he reaches at least third place in the poll. SCP-1025 is currently being worked as a bonus scp for this pack, and it works very similar to @infernalthing idea.

And here are the sprites I mentioned: https://imgur.com/a/h9MjwyB

Here's the idea for SCP-632 ! (Can be used as a foe as well, but with their sizes, it's legit to not be a foe)

SCP-632 : A mini arachnid parasite who goes on brains for placing their eggs and, after being ready to being hatched, forces the host to make their head explode via head traumas against walls, rocks, etc ... The more Isaac advance on the chapters, the more the eggs will be mature, boosting his speed and slightly his power (max 3 times for the strength). Though, due to SCP-632's will to being freed, Isaac will be attracted to rocks and walls, hurting him pretty badly, like one heart ... But spiders will spawn (good and/or bad ones) and will make him bleed !
The "regular" pool (in Golden Chests) will be cool :D

SCP 001-2 The Gatekeeper's Sword

A 6 charge item that summons Uriel (you could use the sprite from the base game) and obliterates everything in the current room. Uriel disappears afterwards. It's basically a reusable Mama Mega except it doesn't destroy everything on the floor when you use it once.

Item pool: Angel (duh)

SCP-035 - Possessive Mask


A 5 charge item (with the sprite of the SCP-035 mask releasing black fluids inside a glass box) that upon activation will throw the glass box and break it, releasing SCP-035 to cause a cause compulsion to nearby enemys to walk towards the mask while the mask leaks a black fluid around itself, dealing damage to those who walk on the fluids, anything that manages to make contact with the mask will equip the mask, replacing their face with the mask l in the " Comedy " expression and becoming a follower until it dies, upon death the mask will fall on the floor and be destroyed, releasing a larger AoE of black fluids, the mask can not work on bosses or major bosses, if the player walks on the black liquids the player will also suffer damage

A item like this would probably be in the Devil Deal pool, maybe in red chests also

i have no idea what the stats will be, but dude SCP-027(vermin king) as an item would be amazing

I wanted to ask to make the SCP-005, but I realised it's already in the game. (Dad's key)

They should make that terrible satellite scp that used tumblr and liked homestuck a boss so we can finally kill it

scp 525 eye spiders
"crying" spiders in room who spawns with some chance by killing enemy
or taking one isaac's eye permanently and its randomly walking in room and shoots at random delay in enemy with character tear
idk maybe treasure\devil pool

I like the idea for SCP-525 to create a familiar that shoots the same kind of tears as Isaac!