A T5 blaze shard can produce a
fair bit of power.
With melee turtles, zombies will drop iron ingot/equipment.
Pigzombie- same again, but for a lot of gold.
Wither skeleton- coal, skulls. (nether star farm with MFFS and frames)
With Gregtech, rotten flesh can produce methane. (you also need enderpearls for the endgame stuff).
I think gunpowder can be back processed, and then made into fertiliser (forestry peat/humus production)
Thaumcraft- rotten flesh makes magic tallow, blaze rods contain praecantatio/fire (can be coupled with charcoal/cobblestone to make unlimited alumentum).