The fight one another in an all-new Marvel event. Considering that multiple characters on each team, including Captain Marvel, Phastos, Spider-Man and Ikaris are fan-favorites, one wonders exactly which heroes will fight when the battle begins. Thus, an itemized list of the best possible matchups is in order, one that would answer the all-important question of who would win in a comic book fight.
The two groups could not possibly be more different. The ranks of the Avengers are mostly comprised of humans, enhanced or otherwise, (though non-human characters like Thor routinely make an appearance). Their home base of operations is Avengers Mountain, but the also operate out of Tony Stark's many buildings and warehouses. Meanwhile the Eternals are decidedly not human, but an artificial race created by the Celestials to protect Earth from the Deviants. Every Eternal shares a common set of powers; they can fly, project cosmic energy from their bodies, and are generally faster and stronger than the average human. Some Eternals choose to focus on using and strengthening one power over others (Ikaris is known for his mastery in the air, Sprite is a master of illusions, etc).
An obvious matchup to begin the fight is Captain Marvel against Ikaris. The two are the powerhouses of their respective teams and their powers appear to be similar (though Captain Marvel is known for projecting energy from her fists and Ikaris from his eyes). Still another vs. fight between popular characters would be a battle between Iron Man and Phastos; both are supremely gifted genius inventors who use their gifts to better the world. But as comic book fans have learned from Marvel vs DC, matching up two heroes with similar powers runs the risk of boring the reader. A truly interesting fight would pit heroes against one another with mismatched powers.
Thor vs Sprite immediately comes to mind; as the brother of Loki, Thor is quite experienced at battling illusions - but only Loki's illusions. This would give Sprite the element of surprise in a fight (but of course, Sprite is considerably physically weaker than the God of Thunder). Another woefully mismatched fight would be the speedster Makkari vs the Hulk in a true test of speed versus strength. Thena, the master at armed combat, could be matched against She-Hulk, who uses no weapons except her own fists. Finally, Captain America could be pitted against Druig (who is much more powerful in the comics than in the film, and considerably less heroic). Rogers has no way to block Druig's mind controlling abilities with his shield, and would have to find another strategy to defeat the Eternal.
Regardless of potential matchups, the Avengers have their work cut out for them. The Eternals cannot actually die, and while the Avengers are very reticent to escalate a battle to the death, the possibility simply does not exist for them in this fight. The Avengers and Eternals are coming to blows very soon - the only question left to answer is the way in which each hero will be defeated.
