The Avengers famously first assembled in the comics to combat the threat of Loki. But the villain the Avengers faced immediately after Loki proved to be an even greater threat, yet in the years since his debut in 1963 he has only ever faded further into obscurity ever since. The Space Phantom may be one of the deadliest threats the Avengers faced early on, and if he were to appear in the MCU the heroes of the silver screen would not stand a chance.
In Avengers #2, the Space Phantom quickly proved the usefulness of his powers in disguising himself amongst humanity. By gesturing toward a passing civilian, the Space Phantom could shunt them off to the other-dimensional Limbo while taking on their shape. He soon used the civilian guise to infiltrate the Avengers' headquarters and immediately proved how dangerous he was to the Hulk and the team.
In the Hulk's guise, the Space Phantom developed a perfect simulacra of Hulk's strength. As the other Avengers confronted him, the villain proved his powers worked just as readily on the artificially enhanced Giant Man or the technologically empowered Iron Man. Throughout his fights with the various Avengers, nobody ever really gained an upper hand on the Space Phantom. His defeat ultimately came due to a fluke power interaction. In trying to shunt away Thor to take his shape, the Space Phantom's powers reflected off the Asgardian's unique physiology so that he accidentally shunted himself off to Limbo.
Despite that niche interaction, the Space Phantom's abilities still stand out to this day as astonishingly powerful. In one fell swoop the villain could both remove the most powerful members of the super team from the field of battle while adopting their powers for himself, meaning that there is almost no threat that could be thrown at the Space Phantom that would not simply make him stronger. Perhaps most chilling of all is that over the years it turns out the Space Phantom the Avengers originally faced is not alone -- there are other Space Phantoms out there with identical powers, making them almost impossible to tell apart.
Though rarely utilized, the Space Phantoms intermittently pop up in the comics in various roles. They have served as agents for numerous supervillains, going up against the Avengers at various points or facing cosmic heroes like Captain Mar-Vell. But if a Space Phantom were to show up in the MCU, there is little chance the film's weaker version of the Avengers would stand a chance.
Bear in mind that the original Avengers of the comics only beat the Space Phantom because of a fluke power interaction. The MCU's Thor is currently not even with the Avengers, but his unique Asgardian physiology does not seem to share in the potently magical basis of his comics counterpart.
Whether throwing the Hulk or Captain Marvel against the Space Phantom, all the Avengers would find are their strongest members missing and themselves forced to fight against the power they formerly had. Especially as the MCU plunges into other dimensions and ties in characters like Immortus, the time may be ripe for the Space Phantom to appear. And if and when that happens the silver screen Avengers better beware.