WARNING: The following article contains spoilers for Amazing Spider-Man #74, on sale now from Marvel Comics.
After a decades-long rivalry, it was finally revealed why Mephisto has targeted Spider-Man for years to undo the good parts of his life and destroy him in the process. After his latest long-term plan involving Kindred failed, the Lord of Hell revealed the reason behind his fixation with Spider-Man. He knows his destiny that he will rule over the Earth and quite possibly the universe. Marvel's devil can see victory on the horizon, but he can also see defeat and the one who brings it: Spider-Man or alternatively, his daughter. Essentially, Mephisto's efforts against Spider-Man are ultimate to prevent his future defeat. But one important member of the Spider-Family was notably missing from his vision: Miles Morales.
In Amazing Spider-Man #74, by Nick Spencer, Christos Gage, Marcelo Ferreira, Mark Bagley, Ze Carlos, Dio Neves, Carlos Gomez, Ivan Fiorelli, Humberto Ramos, and VC's Joe Caramagna, Mephisto's vision seems to indicate that Peter Parker or someone from his direct lineage is going to be responsible for his future defeat. The Lord of Hell has been thwarted before, but he always seems to take it in stride. The identity of the hero who defeats him was revealed, but not what happens in the aftermath of the battle, making it entirely possible that this time Mephisto is destroyed for good. But if someone from the Spider-Family who represents responsibility and kindness is responsible for his defeat, then Miles should be included as a possible threat.
Spider-Man and Mephisto are polar opposites, with one representing the goodness of mankind and the other all of its evils. But Peter Parker is not the only web-slinger capable of representing the goodness of humanity. A lot of newer heroes have risen up to take on the spider mantle and help keep the neighborhood friendly, with Miles being the foremost among them currently. In many regards, he has improved upon the concept of Spider-Man, being much more approachable to people than Peter ever was at his age.
It can be argued that Miles is not Peter's child biologically and thus could be taken out of the running if only someone of Parker's bloodline is a threat to Mephisto. But in many ways, he is the spiritual successor to Peter. There aren't many people that Peter would trust to also use the Spider-Man title, as shown by his recent hesitancy to let Ben Reilly, his actual clone, use the title alongside him. Peter both helped train Miles and was the inspiration for Miles to become a hero in the first place.
Since then, Miles has made a name for himself not as "the other Spider-Man" but simply as Spider-Man. He is just as much a paragon of responsibility and human kindness as Peter. He is as capable as his mentor, entrusted by Parker to patrol New York City while he acted as Spider-Man globally. And yet, surprisingly, he wasn't included in Mephisto's vision of defeat.
But perhaps there is another explanation. Mephisto is powerful, dangerous and intelligent, but he is not omniscient. He has been defeated before simply because he underestimates the strength and resilience of the human spirit. He's fixated upon Peter Parker and his family because of a vision, but that doesn't mean Peter is the only one capable of defeating him.
If Mephisto were to encounter Miles, there stands a chance that he could see Miles as a threat as well. He'd basically be dealing with another Peter Parker, someone as equally dedicated to doing good as the man the devil has come to secretly fear.