I used to do this quite often and I still did it as a thought experiment on my blog. Basically keep the same theme, flavor or character type but swap out Marvel, DC or whomever the original is for a different brand or type.

The Defenders:

Doctor Fate
Aquaman
Orion
and replacing Hulk- I don't know. DC didn't have a direct equivalent of Hulk. Maybe Kalibak? But he's way too villainous.
Some typical suggestions are Blockbuster, Solomon Grundy or Loose Cannon. I think I'd pick Loose Cannon.

Extending the team:

Demon Etrigan (Gargoyle)
Wonder Woman (Valkyrie)
Red Robin (Nighthawk)
Catwoman (Hellcat)

The terminal New Defenders added mutants to the mix for no good reason. DC swapouts suggestions:

Angel = Hawkman
Beast = Congorilla

This is a DC television series in the Arrowverse. Time travellers who are for the most part either antiheroes or screwups.

Swapping them out to Marvel versions:

Rick Hunter: Doctor Who or Mister Fantastic (Doctor Who is unquestionably in the Marvel Universe, predominantly the Fourth Doctor)

White Canary / Sarah Lance: Colleen Wing
Heatwave: The Melter
Captain Cold: Blizzard
Firestorm: Jack of Hearts
Hawkgirl: Wasp
Vixen: Squirrel Girl
Steel: Colossus
Atom: Ant-Man II
Doctor Stein: Olivia Octavius

John Constantine: Brother Voodoo

Director Ava: Natalia Romanova

JSA: All-Winners Squad

Vandal Savage: Comte de Saint Germain
Damien Dahrk: Silver Dagger

West Coast Avengers via DC:

Hawkeye: Green Arrow
Mockingbird: Black Canary
War Machine: Batwing
Tigra: Ubasti
Hank Pym: Dabney Donovan
Wonder Man: Animal Man

Bruce Tim's & Dwayne McDuffy's answer:
Justice League S2 EP 14 & 15
Justice Unlimited S1 #8 the return, #11

he is totally doing Marvel's defenders!
[www.thewatchcartoononline.tv]

opens with Hulk, I mean Solomon Grundy, fighting the US army (complete with Gen.Thunderbolt Ross analogue).
Namor, I mean Aquaman, intervenes.
Clea/Wong, I mean Inza, greets them, and calls upon her lover,
Dr Strange, I mean Dr Fate

There they are, the first/big 3

It won't take that long, but when Hulk/Grundy calls Hawkgirl "Bird-nose", you cannot possibly miss classic/early Defenders any longer, if you could even make to the opening credits before knowing/noticing...that they also face lovecraftian horrors from beyond is also classic/original/starting Defenders.
"excuse me, hawkgirl smash."

JLU S1 E8
Here comes the Silver Surfer! I mean, Amazo in gawd-mode. Amazo joins the "Defenders" & Nighthawk/Valkyrie (some might add angel, but not really) AKA: Hawkgirl is revealed as one, at the end.

Great stuff!

Aquaman's imperious tone can't not be compared to the Sub-Mariner's, either.

It was a great episode.