Spider-Man: No Way Home sets up a universe where MJ isn't an option for Peter Parker, but Black Cat's debut in Spider-Man 4 could solve this problem. Through the first three Spider-Man movies in the MCU franchise, Tom Holland's Peter has always had a love interest, be it Liz or MJ, so it stands to reason he will have one again in the next Spider-Man solo outing. In Spider-Man: Homecoming, Peter has had a crush on the character Liz for so long that his best friend Ned pushes him to talk to her. The arc of the story in Homecoming features Peter slowly gaining the courage to ask her out, only to disappoint her at every turn, which is par for the course in a Spider-Man story. Though they do go out eventually, Liz has to move away due to her father's actions, so her character journey is completed by leaving Peter Parker's life.
At the very beginning of Spider-Man: Far From Home, Peter sets his sights on MJ. He doesn't skip a beat when making a plan to sit next to her on the plane ride for their school trip. The first dialogue that Peter has in the movie is outlining this plan, which involves sharing earbuds and presenting MJ with a black dahlia necklace, which is her favorite flower. He's taken the time not only to learn more about MJ since the events of Spider-Man: Homecoming but seems to have completely forgotten about Liz. From the point that MJ discovers Peter's secret identity, she is his main love interest.
In No Way Home, MJ and Peter are all-the-way dating, up until Peter's impossible decision to save the world. At the end of the movie, he has Doctor Strange cast the spell that makes everyone forget he is Peter Parker, even those closest to him. This makes Ned and MJ into strangers for Peter, and when he visits them to see how they're doing at the end of the movie, he decides that they're both better off without him because it puts them out of harm's way. It stands to reason that in Spider-Man 4, Peter will be dealing with nobody knowing who he is, taking on a life of anonymity to protect the people he loves. However, he will need someone in his corner. This is where Black Cat comes in.
Considering all Peter lost in No Way Home, it's only natural that he will turn to someone who he cannot pose a danger to as Spider-Man. Since he gave MJ amnesia, Betty Brant already exists in the MCU and has a relationship with Ned, and Gwen Stacy was well-covered in the Amazing Spider-Man movies, the looming next love interest for Peter Parker is none other than Felicia Hardy herself, the Black Cat. This would be the easiest and clearest way to justify the amnesia spell and to help Peter Parker move forward as a character.
Although not super-powered, Black Cat is a super-anti-hero who Peter doesn't have to worry about hurting through his exploits as Spider-Man. She's used to dealing with villains and is an established love interest of his from the comics. If Spider-Man 4 wanted to put Peter in a relationship with someone other than MJ, this would be the one that makes the most sense given the events of the previous three Spider-Man films. It supports Peter's conviction of not putting anyone he cares about in danger ever again and makes his life-changing decision in Spider-Man: No Way Home all the more believable.
