Doc Savage Vs The Spider (a hero that does not kill meets the hero that kills)
The Spider hunting down the criminal called the Green Hornet (it would have to be prior or in a different continuity than Masks)
Jungle Lord meets the Phantom (it seems Hermes mini is giving us this one unofficially)
Flash Gordon meets Buck Rogers (DE might have missed this opportunity when they had both)
Warlord of Mars meets Dr. Who against the Ice Warriors of Mars
Warlord of Mars meets the Manhunter from Mars
The Phantom meets the Batman (DC missed this opportunity)
John Carter: Warlord of Mars meets Killraven: Warrior of Worlds
Jungle Lord meets Turok: Dinosaur Hunter
Mandrake the Magician meets Vampirella
Jungle Lord meets Magnus: Robot Fighter (shame Russ Manning is not still alive)
Mandrake the Magician looks through the Occult Files of Dr. Spektor
Dr. Spektor meets Vampirella
Lone Ranger/Sgt. Preston/Green Hornet generational crossover (all three created for radio by George Trendle)
The Spirit/The Phantom/The Shadow
Bionic Man/Bionic Woman meet the Terminator or Robocop
Bionic Man/Bionic Woman meet Magnus Robot Fighter
Jungle Jim meets Turok Dinosaur Hunter
Miss Fury meets Catwoman
Miss Fury meets Black Widow
The Spider meets the Punisher
Doc Savage meets the Fantastic Four
Doc Savage meets Iron Man
Sherlock Holmes vs Fu Manchu (there was a book about that meeting--maybe a sequel or adaption can be done?)
Fu Manchu vs Charlie Chan --DE does not lease either of these yet-- fictional Chinese hero detective vs fictional Chinese villain
Charlie Chan and Mr. Moto--DE does not lease either of these yet
John Carter meets other ERB cosmic heroes (from ERB's Venus, Moon, Beyond the Farthest Star, etc.)
Red Sonja meets Claw the Unconquered again (maybe to tie up the cliffhanger from Wildstorm's Claw series)
Mandrake meets Zatanna or Zatara
Justice Inc meets Masks meets King's Watch meets the Gold Key adventurers