The first thing that comes to mind, when talking about another MCU Hulk movie, is the cast. It's quite odd to have both Ruffalo and Norton cast as Bruce Banner, and so it's hard to choose which tone the film should choose: Incredible Hulk Sequel or Avengers Spin off? I say both. Create a film that keeps both Banners in continuity, while at the same time, explaining the difference between Norton's and Ruffalo's Banners.
The film would largely consist of Ruffalo's Banner now working for SHIELD's gamma division. A place where Gamma related villains, such as Gremlin and Abomination, are stored, and Gamma related heroes can work. People like Rick Jones, Doc Sampson, She-Hulk,Betty Ross and Samuel Sterns can be introduced as key members in the Gamma division. Everything is well and good, except that General Thunderbolt Ross has been put in charge of this small organization, for his "success" in dealing with Blonksy. Banner is told that he been brought in to deal with "Red," another Hulk like beast who has been terrorizing their Bunker, SHIELD feels like a member of the GAMMA division is responsible for "Red," so they need an outside opinion to solve the mystery.
Eventually, Ross would leave the Bunker for an "emergency," and low and behold, Red shows up. Bruce and Sampson go to investigate, but before Bruce reveals, he physically can't get angry enough to transform, so Doc give him a drug that could force him to transform. Red wipes the floor with both of them,all the while asking where Blonsky is. He flees the scene. At this point Bruce begins to discover that Ross is Red. Just as Bruce and the group are ready to confront Ross about this, Ross is killed by Red. Bruce takes the drug again, and fights Red once more. Red defeats all of them, except for Sterns.
Just before Red kills Banner, he-Hulk manages to fire a heavy sedative into Red, forcing back into human form. This is where it is revealed, Red is actually Edward Norton's Bruce Banner. This where Bruce (who still can't become the Hulk, and has no drug to induce himself) begins to demand answers. Sterns and Banner 2 explain: Banner 2 comes from an alternate universe where Betty died from internal injuries shortly after Harlem. This Bruce 2 became so enraged after her death, that he became the "World Breaker Hulk." As his Hulk persona took over him more and more, Bruce 2 became redder and redder. He was forced off the planet by the Avengers, but came back in World War Hulk fashion. Sterns came into contact with him while looking through parallel universes for a better version of the Hulk, Bruce 2 agreed on the condition that he could kill Blonsky and Ross all over again.
Bruce 2 becomes Red, and in effort to force Bruce to become just like Red, kills Betty. Bruce becomes the Hulk, and is becoming more and more enraged as he keeps missing hit on Red, all the while, Red is taunting him, making in worse. Hulk manages to overcome Red, but the Leader escapes with Abomination and Gremlin. It all ends, with the Hulk going "gray" as his soul died with Betty.