The trick is the MCU would never make a Deadpool movie. He might get to go to Netflix, but even then he would have to sit on the bench every other year and wouldn't get to interact with the other X-Men.
Similarly, I don't think Feige would let the X-Men get as dark as they sometimes have in DOFP or dwell so much on the Holocaust or Cold War as they have done in previous films. Also, the "in the closet" subtexts would probably be dropped.
It is a tradeoff. I honestly am in no hurry for a reboot. But if they could figure out a way to explain realities merging for a crossover film? Sure. But I would want it to be after both Infinity War and apparently a cosmic Phoenix Saga (where the X-franchise is supposedly going after Apocalypse).That could be fun, though very unlikely.
I would agree with Deadpool, only now that it's been made and has been shown successful I think Marvel would be smart enough to let it continue to do it's own thing. However there is a difference between "doing it's own thing" and "using our toys". No chance Marvel would let Deadpool interact with an underaged Spidey in a meaningful way. They might allow passing references or a quick, single joke appearance but nothing that would be advertised as "including Spider-Man". And obviously any character they did use would come with a list of restrictions and their depiction would have to be in line with the rest of their character's appearances. So even if we got, say, Tony Stark in there it's not like he's going to be swearing and drinking and naked just because he's suddenly in an R rated film. He's going to be the same Tony as the other movies, just with Deadpool to make fun of him.
Plus, honestly, Deadpool isn't the best candidate for a Netflix series. It's not simply about being more "mature", it's also a choice of format. Netflix is very much in the spirit of limited-runs or collected trades where the in focus is on a single story arch, but each issue leads into that. The movies, then, are single issue events with that fun romp factor and the spectacle, but also the plot needs to be more self contained. There's potential for a character to do either, sure, but Deadpool is a character that really plays more towards the later. Although if he were absorbed into the MCU Deadpool would probably have the highest chance of any film of featuring the Netflix characters.
Although I also feel like the MCU outside of Netflix deal with a lot more mature subtext than it's generally credited for, it just tends to be overlooked or dismissed by a lot of people because of the lighter tones. Which is unfair, because mature isn't necessarily the polar opposite of lighthearted any more than heavy gore really equals mature. Besides that, though, the first new inhuman they find on Agents Of SHIELD happens to be a gay character who directly compares coming out of the closet to being open about his inhumanity so it's hard to say their mutants would completely erase that aspect.
Still, I really don't see a crossover working. It' just so many characters with so much information you would have to set up that it could only work as pure spectacle, and while a niche group of fans might love that and not question the inevitable cameos-in-place-of-characterization and plot holes wide enough to fit an alternative universe through I don't think you could sell the resulting product to a wide enough audience to justify the cost. And personally I just have no interest in seeing a film that's all flash (and likely not even well executed flash).
