Just finished reading Michael Moorcock's Swords trilogy. Poor old Corum-The Eternal Champion spends the books flashing from plane to plane. He finds many enemies that he has no prior knowledge of,but has allies of great power who help him to win against a triumvirate of Chaos.
As a wargame it could be like a tournament. One side keeps their core heroes but has a random fantastical auxiliary. The enemy is led by a known personage but their forces are randomly drawn.
Think:
Mediaevals against gargoyle host,Hope they have archers, Fighting on a burning plain.
Gallic types against undead in an endlessly shifting city scape.
Flying boats against ancient flame-cannoned city.Find the Chaos ruler and dispel them.
The Heroes live or die, are taken prisoner and need rescuing from another plane or are flung into another world in the middle of a another battle at random.
In Corum's case he can summon all the enemies he destroyed in the previous battle to aid him.
Fantasmagorical terrain would help creating bizarre battle grounds to fight over.
Good books worth the read. And then there is Elric's world.
Chaos ships that sail through dry land.