So far I thinking
deathstroke and hush
psycho pirate and joker

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The Hal Jordan and Kyle Rayner versions of Green Lantern (the experience of Hal Jordan, the imagination of Kyle Rayner and the combined willpower of BOTH of them)

Batman and the Barry Allen Flash (somebody with Batman's training and gadgets combined with the Flash's speed would be incredibly hard to beat - especially since Bruce probably wouldn't be playing the sort of games Barry tends to)

Supergirl and Wonder Woman (a female Kryptonian survivor who landed on and was raised on Themyscira)

Black Lightning and Steel (Black Lightning's electrical powers mixed with Steel's scientific skills and powered armor)

Vandal Savage and Ra's al Ghul (two of the DCU's most ambitious immortals)

Superman and Captain Marvel (young Clark Kent gets chosen to be the champion of Shazam giving him not only double strength versions of his strength, speed, invulnerability, intelligence and flying ability, but it would also make him immune to Kryptonite, give him at least a few powers that he ALWAYS has even under a red sun and give him an increased resistance to magic in addition to at least fifteen years MORE experience as a hero if Clark got his start as a hero when he was a kid)

General Zod and Darkseid (two incredibly powerful cosmic tyrants merged into one being)

Zatanna and Raven

Lex Luthor and Doomsday (imagine how dangerous somebody with both Lex Luthor's intelligence AND Doomsday's physical strength and resurrective/adaptive immortality would be)

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what do you think of teen titan combine with the outsiders?

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Yeah. I could see that (with the aforementioned exceptions of Black Lightning being fused with Steel and Raven being fused with Zatanna):

Aqualad getting fused with Geo-Force (a prince with power over both water and earth, being able to survive comfortable above and underwater, telepathic animal control, massively superhuman strength and durability, gravity control, senses heightened enough to be able to comfortably operate in deep waters, able to fire powerful blasts of lava from his hands - yeah, definitely not somebody any sane individual would want to mess with).

Nightwing and the Wally West Flash (I mean, if Batman is fused with the Barry Allen Flash, then it makes sense to fuse their respective first crime-fighting partners too)

Starfire fused with Halo

Wonder Girl fused with Katana

Beast Boy fused with Metamorpho

Having trouble thinking of an Outsider to fuse Cyborg to, though.

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I feel halo and raven
Starfire and windfall
Cyborg a tough one though

Yeah. It is. And most of the other cyborgs (small "c") in the DC Universe are evil. Not all of them, admittedly, but most.

Oh, well, might have to go with a gender-flip and fuse Cyborg with Indigo/Brainiac 8

Another one: Gorilla Grodd merged with Bane (Grodd's strength enhanced by Venom, plus Grodd's telepathy and telekinesis AND the combined intelligence of both Bane AND Grodd would definitely be a nightmare for most heroes)
 
Hugo Strange merged with the Riddler

Captain Cold merged with Mister Freeze

Doctor Death merged with the Scarecrow (Doctor Death's skill with poisons, Scarecrow's psychological knowledge and the combined chemical knowledge of both of them)

Heat Wave merged with Firefly

The Monk merged with Clayface (they're both shapeshifters [though the Monk's shapeshifting is limited to wolf, bat or mist] who possess limited superhuman strength and some invulnerability to most mortal weapons, but Clayface's greater versatility on his shapeshifting mixed with the Monk's hypnotic powers and greater experience would definitely be a formidable combination),

Killer Croc merged with the Man-Bat (Kirk Langstrom trying to use a formula that combined a bat's enhanced hearing with the limited healing factor of some reptiles and ending up getting mutated into a humanoid mix of a bat and a crocodile [with heightened senses, medium-level super-strength and super-toughness, flying ability, massively powerful jaws and other abilities] would definitely be some fantastic nightmare fuel)

Doctor Sivana merged with the Parasite (Sivana's scientific genius mixed with the Parasite's ability to drain abilities and memories with a mere touch would definitely make for a scary villain)

Black Adam merged with Sabbac (anybody who had the combined powers of six Egyptian Gods AND six high-level Demons would be a nightmare for almost any hero with half a brain)

Titano merged with Beppo the Super-Monkey (a Kryptonian powered monkey permanently enlarged to King Kong size would definitely be incredibly formidable)