hmmm..this would be some very neat shizz, since there is basically NO USE for zombie flesh...
in my opinion i think it should be like the wither where u craft it with blocks but first
Make zombie flesh blocks out of 9 zombie flesh like other blocks....and maybe in the furnace (to make it more complex) put a wither skeleton skull on the top of a furnace and as the fuel, maybe a stack of zombie flesh to make a zombie head... (just a thought

) and then u have to like craft it in midair (like a wither).
crafting recipe:
for the head [2x2x2 flesh blocks of zombie] and then place 4 zombie heads on the top of the zombie blocks.
then for the middle of the body (2 [thick] x 3 [wide] x 3 [tall]) making the 2 center blocks of it 2 dia blocks. then the outer edges, zombie flesh blocks. and then a 1 zombie block connecting the bottom to the top as well as one connecting the body to the head. and hands made of zombie blocks
[addition]: 5 gold blocks in the right hand which will make his sword
Feet made of 5 zombie flesh blocks like a upside down "U" (x2) |---| connected by lapiz blocks (7) like an upside down U but |-----| [Connecting the two sets of feet]
and then when u complete the whole process. it turns into a huge mutant zombie with one gigantic head! with a super gold sword in his hand.
Stats [for the "mutant zombie" which should have a name"] A SHIZZ TON OF HEALTH!!
A super sword {made of the gold blocks}
regeneration every X seconds and during this it takes lower damage from the player/s
and maybe a shield around it? [/ONLY\During the process]
Attack: Maybe u can only attack with a certain weapon {fits same criteria like a slider}
Rewards: A lot of XP
and a chest with randomly generated goods.
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Sounds like an awesome IDEA!!! Woot! and i wanna see some more powerful mobs here soon!
keep it up!
My thoughts ~Slash