This mod modifies all characters and adds 3 items to allow them to begin with a transformation (except Eden because he's so random and Keeper because he's character 13 and there's only 11 transformations).

This is a list of the changes made:

*Isaac = Yes Mother? (because it's Isaac's mother) = Mom's Lipstick, Mom's Underwear, Mom's Heels
*Magdalene = Seraphim (because most Seraphim items give heart containers) = Rosary, The Halo, Guardian Angel
*Cain = Spun (because... gamblers do drugs I guess? It's a dump transformation, ok?) = Growth Hormones, Synthoil, Roid Rage
*Judas = Guppy (because Judas has 1 heart container and Dead Cat causes that anyway) = Dead Cat, Guppy's Collar, Guppy's Tail
*??? = Bob (because they're both corpses) = Bob's Rotten Head, Bob's Curse, Ipecac
*Eve = Conjoined (because Conjoined is hard to use and I really suck at playing Eve) = Rotten Baby, Mongo Baby, Little Steven
*Samson = Fun Guy (because dump transformation) = Thin Odd Mushroom, Blue Cap, Magic Mushroom
*Azazel = Leviathan (because demons) = Pentagram, The Mark, The Pact
*Lazarus = Oh Crap (because he's shit) = Flush!, E. Coli (Lazarus cannot find The Poop during a run, but does not start with it)
*Eden = no changes
*The Lost = Beelzebub/Lord of the Flies (because it's a terrible, terrible flight pun) = Skatole, Mulligan, Hive Mind
*Lilith = Super Bum (because he's a powerful follower) = Bum Friend, Dark Bum, Key Bum
*Keeper = no changes

This mod was made using the help (aka blatant stealing) of the Isaac Character Editor
The numbers on the left side of the screenshots are from Missing Hud 2. It is not a mod on Modding of Isaac, but that's what Google is for.

TURNS OUT THERE'S A MORE BALANCED MOD CALLED THE TRANSFORMING OF ISAAC THAT DOES BASICALLY THIS BUT SLIGHTLY DIFFERENT OOPS
Well if you want to be stupidly powerful or prefer my changes for whatever reason, this is the mod you'll want. Mine isn't designed to be balanced, it's designed to give you the transformations.