Wade Wilson a.k.a. Deadpool is one of the most unique Marvel characters to exist, as he is aware that he is living in a fictional comic world and constantly breaks the fourth wall because of it. There have been discussions of Deadpool's integration into the larger Marvel Cinematic Universe, starting with a third Tom Holland Spider-Man film rumored for release in 2021.
With the first "Deadpool" making over $700 million and eventually becoming the highest grossing R-rated film as well as "X-Men"-related film of all time, there is no way Disney does nothing with the character. While the X-Men or Fantastic Four may need a few more years for audiences to accept new versions of the characters, Deadpool is in his prime for use.
Currently, in the MCU, Peter Parker seems to be in constant need of a father figure to further mentor him into the great hero he will one day become. "Spider-Man Homecoming" had Robert Downey Jr.'s Tony Stark as the mentor that was similar to the cool uncle, and "Far From Home" will have Samuel L Jackson's Nick Fury as the stern stepfather with a completely different mindset, who will likely not have any time for a lot of Peter's BS.
The possible introduction of Deadpool allows for a very interesting dynamic between Spidey and Wade, not just that they both wear red and make jokes, but their separate moral codes. Unlike Spider-Man's relationship with other heroes, Deadpool does not make Peter better as a person, but potentially worse.
Deadpool is a narcissistic hired mercenary that kills people for a living while Spider-Man leaves his enemies alive for the police and has only used his suit's instant kill mode once on aliens. The conflicting ideologies would be the core of the relationship. Deadpool would be the perfect anti-mentor and bad influence to have for this young, naive Spider-Man, who still has to learn who to trust and who to not pay attention to.
Deadpool's intentions may be good at times, but for the most part, he is a maniac who occasionally finds joy in the people he dispatches. Deadpool is essentially what Spider-Man could turn into as an adult if he goes down the wrong path. It has not yet been confirmed if the continuity of the first two "Deadpool" films will carry over into the MCU (Disney usually pretends to go with a clean slate), but the end of "Deadpool 2" saw Wade's character start to become accepting of the idea of mentoring children.
Spider-Man can be that young superhero that Wade needs to soften him up a bit and to allow for that storyline to continue (plus that PG-13 rating). As long as the core essentials of the character remain intact, a more PG-13 toned-down Deadpool could definitely work, especially when sharing the screen with other heroes.
But, when it comes to most superhero team-ups, there is a good chance that the heroes will end up fighting before becoming even reluctant allies. With such large personalities in the same room together, there is bound to be friction. Tom Holland and Ryan Reynolds are the two of the best cast actors of this superhero renaissance, becoming major fan favorites almost overnight.
With arguably the two funniest characters in the Marvel Universe that are yet so distinct along with that Marvel brand of humor would make any movie memorable.
