Will the Defenders / Frank Castle ever show up in MCU?

I think after Avengers 4 is when the chances for The Defenders and The Punisher to appear in one of the movies go up, the percentage is variable depending on who you ask but still it'll rise. After Avengers 4 the MCU will enter into a "new age" where other characters begin to be more integral to the bigger picture. I'm gonna take a guess that like with the movies being trilogies each of The Defenders' individual series will probably be three seasons each then, like with characters such as Iron Man, they can bring them in to other shows and movies with varying levels of narrative importance. Based on how The Defenders ended I think the focus from here on out in the individual shows will be about them to be more willing to get out of their comfort zones( street level crimes) and face larger threats. This I see being emphasized with Jessica Jones( Who still has issues using the full extent of her powers and doesn't like to be lumped together with other people with abnormal abilities) and Luke Cage, Daredevil a bit less his main arc is obviously going to get his life back together - Danny already has the most exposure to the fantastic, he just needs to define himself further and aim to be a more legitimate superhero. When all the characters get "settled", and their third and final seasons are over I'd say its a good time to finally being them into the movies and other shows if necessary. Each of the four Defenders have Avengers ties, so if they want they can bring in one or two or all four of them if they'd think it would serve the story good to have them. There's a lot you can do with these characters for the movies, but again I see them mostly being supporting characters in ensemble pieces( at first, perhaps as time goes who knows?) but with larger roles in TV appearances. The Punisher I'd like to see acknowledged in the movies, you don't necessarily need to show him but a news brief or some reference to him in passing. Castle doesn't really work the best with others and his best stories are usually where its all about him basically, it would be cool if in the supposed Thunderbolts movie that is in development he gets a brief cameo as a potential candidate for the team but is deemed too much of a risk.

The Punisher doesn't need to be in the MCU. Only if Disney have the balls, to make an Rated-R movie with Jon Bernthal. But we all know, that Disney doesn't have ball, and will Castle watered down, to the maximum.

Right, because Klaw getting his arm ripped off was tame. So was Ronan smashing a guys' head open so he could bathe in his blood, or Bucky killing Mr and Mrs Stark, etc.

I doubt anything Punisher does will be that extreme. He just kills people with guns or knifes them or tame stuff like that.

Yes, because Disney has never ever released an R-Rated movie before....*Sarcasm*

Because there was no humor whatsoever in The Defenders...With the exception of some violent imagery it's not that hard of a TV-MA series and could've been edited down to a TV-14 without losing much.