I like the Mummy/Pharoah concept, and I also like Arondight's Reinfield idea, so I thought I'd throw out some others.
Wood Golem becomes
Fire Golem when targeted by Firelance/Firestorm
Fire Golem becomes
Ash Golem when targeted by Ice Bolt.
Ice Elemental becomes
Steam Elemental when targeted by Firelance/Firestorm
Steam Elemental becomes
Ice Elemental when targeted by frozen/congealed
Corpse becomes
Frankenstein when targeted by Lightening
Clockwork Golem becomes
Robot when targeted by Lightening
Ghost becomes
Reborn Soldier when targeted by Holy Light
Plagueling becomes
Giant Rat when targeted by Holy Light
Giant Rat becomes
Plagueling when targeted by Alphatoxin
Sand Elemental becomes
Mud Golem when targeted by freeze/congeal
Dr. Jeckle
becomes
Mr. Hyde when targeted by Rage Elixer
Flesh Golem becomes
Stone Golem when targeted by Stone Skin
Chaos Mist becomes
Iron Sphere when targeted by Gravity Pull.
Iron Sphere becomes
Chaos Mist when targeted by Chaos Seed/Chaos Power
This concept opens a large dynamic to the elements game. These are just ideas to get us all thinking.
Jallen