Legendary Substitutes

Forte - Seabrand Dragon is surprisingly adequate, Genesis Dragon is also a slightly less budget option.
Merlin - For each Merlin you would normally use, substitute a Scholarly Witch AND a Spectral Wizard.
Moon - Gilgamesh
Tsubaki - Swordsman, other removal
Otohime - Noble Knight
Aurelia - Goblinmount Demon, Frontguard General, other Ward followers
Tia - Elder Treant
Olivia - Leonidas
Cerberus - Aggro: Soulhungry Wraith, Control: Charon. These aren't so much substitutes as curve-fillers.
Cannot be substituted:
Ancient Elf