hear me out, isaac should be dungeon master. canonically everything is either just in his mind as he is suffocating in the chest or a story made by him and his father and he also made up the legend of bumbo.i know that this idea is probaly complicated and maybe hard to make but awesome

stats same as isaac but litle bit smaller to balance things out
also he would have 3 broken hearts

he would start with one item with 2 gimicks! it would be called dice set(i know, very creative) and it would have 5 slots you change by pressing ctrl key. 3 slots would be dices and 2 would be special.
dice would mimic any other dice items. it could mimic d100, spindown etc hopefully even some special, unique ones
if you clear the room it will charge 1 random dice and if you kill an enemy you have a small chance to charge random dice. however if you take damage, dice you have equipped (or random one when you hold the special cases) gets reroled.

the special slots would be linked to secend gimick, the cosplay ability because dms have to roleplay most characters since players need other people to interact with and also all the other characters are either versions of isaac or just him cosplaying
first slot would be "swich" between 2 modes.
first mode would be changing role when isaac takes damage
second mode would be changing role when he clears the room
each vanilla / b side / c side characters would have their role in this item.
isaac would take both negatives and positives of character he takes role of.
Examples :
if he takes forgotten his health would be temporarely converted to bone hearts but he would get the melee atack.if he turns into judas he would get more damage but less health. if he turns into tainted isaac he would have limited number of items he can take but he would be able chose between 2 items (the items would be limted only to this role, other ones would be smelted)

second slot is for role's item. basically starting active item of character he turns into gets into this slot. if role has none, its empty

The Lost as an assassin with mom's knife and dark arts