Marvel has the rights to Punisher and as of yet, have no plans to use him. I'm saying that's stupid. The Punisher is a character with allot of depth, and meaning. He suffered such loss even Batman can't match his pain. His entire family was murdered by the mob and that made him start a one man war on crime. So why don't they use such a cool complex character? It hasn't worked very well in the past. The reason the previous attempts haven't worked, is because they made the movies about the violence, and not about the characters. Thomas Jane was the best Frank Castle we've gotten. He wasn't about senseless violence like Ray Stevenson's, or whatever the hell Dolph Lundgren's was about. Jane's Frank Castle was about Avenging his family at all costs. While that theme was evident in all adaptions of the character, it was not always as important. Whether he was saving kids from being sold into slavery, or repaying a debt to an undercover cop that he accidentally killed. 2004's Punisher was about Avenging, so... why not add him to The Avengers?! His logo has already semi-appeared in an MCU film, [Iron Man 3](movie:24391).
I love easter eggs. So, My chosen plot line would go as follows: The members of S.H.I.E.L.D. that weren't Hydra, have been monitoring potential additions to their ranks. Among those are Matt Murdoch, Luke Cage, Jessica Jones, Danny Rand, and Frank Castle. Frank Castle still at large and fighting crime after the events of 2004's punisher has been on their radar for a very long time. They give him the opportunity to continue his fight legally, with their help. He accepts and becomes Frank Castle Code name "Punisher" [Agents of S.H.I.E.L.D.](series:722469) or whats left of it. He fights along an elite task force of the other people listed above fighting the crimes that happen in the shadows.