WARNING: The following article contains spoilers for Heroes Reborn #2 by Jason Aaron, Dale Keown, Carlos Magno, Scott Hanna, Edgar Delgado, Ed McGuinness, Mark Morales, Matthew Wilson, and VC's Cory Petit, available now.
In Heroes Reborn, Blade the Vampire Hunter wakes up in a drastically altered Marvel universe in which the Avengers were never formed. In the place of Earth's Mightiest Heroes is the Squadron Supreme, Marvel's slightly twisted version of the Justice League, protecting and controlling the new world. While Blade will almost certainly succeed in his mission to gather the Avengers and set things right, the story so far emphasizes the immense power the Squadron holds, which makes it look like DC's characters could defeat Marvel's in a conflict should another genuine crossover ever happen.
Created by Roy Thomas and John Buscema, the Squadron Supreme first appeared in Avengers #85 in 1971. The group is used as a stand-in for the Justice League in Marvel titles, although even the initially heroic squadron has often become corrupt. Arguably the most important member of the team is Mark Milton/Hyperion, the Squadron's equivalent of Superman, who has a similar origin and set of powers as Kal-el. The vigilante Nighthawk is the Squadron's equivalent to Batman and the team also includes Zarda/Power Princess, a reimagining of Wonder Woman, among others.
Hyperion is the most famous and beloved hero of the Heroes Reborn world and Heroes Reborn #2 demonstrates his incredible power. Throughout the issue he defeats a plethora of superhuman opponents including, among others, the Hulk, a Hank Pym/Ultron hybrid, and even Galactus, though he does so using brutal, often lethal methods.
The Squadron Supreme are one of many instances of DC and Marvel indirectly portraying one another's characters, but there have also been plenty of full-blown crossovers between the two companies. Out of continuity stories have allowed beloved characters like Superman and Spider-Man to cross paths many times in the past and some tales, like JLA/Avengers are considered canon on some level. Possibly the most well-known crossover was DC vs. Marvel, which pitted characters from each company against one another in battle. Another famous crossover was The Uncanny X-Men and The New Teen Titans, which united the titular teams of what were at the time their respective companies' most successful series. One of the most unique creations was Amalgam Comics, an imprint that depicted a world full of hybrids of characters from both universes such as Dark Claw (Batman and Wolverine) and Super-Soldier (Captain America and Superman).
Unfortunately, another genuine crossover like these seems unlikely, at least for the foreseeable future, but not necessarily for the reasons fans think. While some fans insist on only being loyal supporters of either DC or Marvel the truth is that most love both, and the same goes for the creators, most of whom have worked for both companies at different points in their careers. However, the companies' places in the business world means that they are competitors in some ways. Because of the massive increase in popularity of superheroes in film and television the comics companies are now much more vital parts of the media empires of their owners (in Marvel's case, Disney, in DC's AT&T's Warner Media). Another inter-company crossover would require a lot of complex negotiating between high-level executives of these entities, who would likely not be interested in sharing the profits the venture would produce.
This is also why any kind of television or film crossover is unlikely to happen, despite the aforementioned popularity of the superhero genre and the fact that both franchises are moving into positions where, from a storytelling perspective, such a thing would be possible. Both the Marvel Cinematic Universe and DC's live-action franchises are embracing the concept of the multiverse. Marvel will do so in What If...? and Doctor Strange in the Multiverse of Madness as well as possibly other properties like Spider-Man: No Way Home. DC has already done so on television, culminating in the Crisis on Infinite Earths event and is set to expand the concept to the big screen with The Flash next year.
Once the concept of the multiverse has been thoroughly introduced to audiences, mounting a DC/Marvel crossover is a relatively easy storytelling task but the obstacle of the owners' cooperation still exists. In addition to not wanting to share the money, executives likely wouldn't want to risk confusing audiences by blending their brands or risk elevating their competitors' profiles as much as such a big event certainly would.
Regardless, it's fun to speculate what a modern conflict between the two universes would look like, whether it took place in comics or another medium. Although Heroes Reborn indirectly highlights the Justice League's power, if a confrontation occurred on Earth, Marvel would have the advantage in sheer numbers due to the presence of the mutants and Inhumans. But going to space might tip the scales in favor of DC, as Marvel doesn't have an equivalent for the various Lantern Corps with their thousands of members.