The aim of this is to essentially present a badly calculated, unnecessary film plan that hopefully after communal suggestions will become a potentially serviceable MCU plot. Expect inconsistencies and plotholes, it’s a very undeveloped approximation of a story that needs further complications. Obviously it won’t get made; this is purely fun.

Shoot it down if you want; I already have.

We all fall off our chairs convinced we've actually been shot when we see this right? Oh, yeah me neither.

Desire is often expressed for a Black Widow solo film. Also, Mark Ruffalo’s Hulk has never been given a solo film. An issue with a Black Widow solo film being set before the events of most other MCU films is that her villainous backstory, outright showing some of the depravities she was apparently complicit in (e.g. the hospital fire that’s briefly alluded to in Avengers Assemble), could decrease audience sympathy and undermine her heroics since that point. The issue with a Hulk solo film is the limitation in fighting choreography, and the limitation of villains who could actually pose a credible threat to him. The only powerful enemies would be similarly sized and powered, which has already been used in one Hulk film as well as several other MCU films. And also rock 'em sock 'em robots, not that there's a film of that—they were in Toy Story 2 briefly, maybe the fourth one can be about them—can't be worse than what they're planning, can it? (That's a genuine question, I have no idea what they're planning.)

It genuinely feels surreal watching Scarlett Johansson playing her MCU role interacting with MCU characters who aren't played by their regular actors. You know the best bit? It's canon. It clearly isn't, BUT Scarlett didn't realise this was SNL sketch footage and thought the film was really happening. She clearly didn't. Apparently, though, a romance featuring her and Hulk has been jovially suggested elsewhere online. I know it'll never get made, but I suddenly wrote on how I envisioned this premise would unfold, and here we are. Assuming this takes place post-Avengers 4, and assuming both survive, the pair attempt to test a relationship, building on the established chemistry from Age of Ultron that there hasn’t been an opportunity to continue since.

A lot of my dates have done that to me, even to the point of saying. "But I need the other guy" presumably hoping I ignore their murder attempts as long as they reference a film. To be fair, they were right. She's lucky he hasn't got complete control over the Hulk otherwise he'd have just died.

Anyway to begin the imaginary romance film:

Bruce Banner could discover that a scientist is attempting to produce a cure for gamma radiation that thus far has proved unsuccessful. The scientist would ideally be directly from Marvel comics, even if none have ever been known to undertake these actions. In spite of failure, the scientist is continuing to produce the serum determined to fix it. Black Widow decides to notify the reformed SHIELD who plan a mission to safely destroy the serum before it can cause any irreversible harm. Bruce Banner decides not to partake in the mission out of fear he’d lose control during it. Bruce gets in touch with Spencer Smythe, in this universe a former colleague of his. Bruce would be asked to assist his son in a project. A lot of Spiderman fans would know Spencer’s son as Alistair Smythe, who later becomes the Ultimate spider slayer.

Here's how he looks in the adored 90s Spiderman series.

Those appendages would need to be robotic and attached to a backpiece, like the shoulder guns on Iron Man's suit- they'd look more hideous than his mullet (which would also need to be robotic obviously) if they were flesh like here.

Alistair would appear in this film reinvented as much younger and playfully affable, and a fan of the Avengers but still an intelligent professional, similar to Peter Parker, his traditional nemesis. Alistair would use leg braces similar to the ones used by War Machine in Infinity War rather than being wheelchair bound given his technological knowledge. After assisting Alistair with a project, Banner would discover he was simply being used. Alistair was gaining his trust in order to eventually convert him into the Hulk to provide a diversion for Spiderman, who he is fighting for reasons that would be revealed in a Spiderman sequel that shows the same situation from a different perspective, in which Hulk would only play a brief cameo role of similar prominence to the one Spiderman would fill in this film. Banner would logically turn back into the Hulk and due to the length of time since his last episode his control would have decreased from what’s been shown in previous films. Alistair uses phone recordings of Black Widow’s from Banner’s phone he pickpocketed from him during the initial transformation to calm him enough that he can follow commands. During a very brief scuffle with Spiderman which will end. The end will possibly be the result of a prototype spider slayer capturing him and flying him away from the area—with the fact its unfished being the explanation for why he isn’t outright killed. The attacks Spiderman received for the hulk could be said to negatively affect his Spider sense and for this to be the reason he simply wasn’t alerted to the spider slayer’s approach. The Spider slayer’s could possibly have even been designed for peacekeeping purposes in this universe like Tony Stark’s blue robots in Age of Ultron with the iconic name being used in a later film.

Unless Spidey Uses Soft Harmless Webbing—Hulk's Crippling Weakness

oh my god the films already happened and I found a screenshot from it.

Hulk opts to attack Smythe for causing his relapse. In his panic to play Black Widow’s recording, he accidentally deletes it, he uses one of his gadgets to create a harmless explosion that briefly distracts the Hulk, and while Hulk is distracted he urgently calls Black Widow, who is still on the mission to find and destroy the dangerous gamma radiation "cure."

NOTE: Black Widow’s mission would actually take up a large part of the film, with her possibly being helped out minimally by another Marvel character. Minimally though, since overuse of them would cause loss of focus. Smythe himself would only appear for approximately 15 minutes, despite being important to the plot.

Smythe would be able to gain contact to Black Widow using Bruce’s phone and plead with her to subdue Hulk with her voice, this would eventually work and after returning to his Bruce Banner form he would briefly confide in Black Widow how he’s worried about the amount of destruction he could cause as Hulk in future now that his control’s slipped. This conversation would presumably happen after Black Widow had taken down several enemies to explain why he’s able to hold a conversation without enemy interruption. After the conversation, she battles the aforementioned scientist and retrieves the final vial of the gamma cure. Black Widow confides in another undecided character that she’s considering giving Bruce the vial after hearing how distressed he was about his relapse, and that she’ll assess whether the risks of the cure outweigh his current issue when they’ve been reunited.

Oh my god, this is the perfect picture- suspicious the film's already been made now.

The final scene would show them being reunited, with Black Widow advising Bruce that he shouldn’t try to cure himself unless it has the opposite effect and that experimentations in past films (like his and Tony’s attempts to put an armour suit around the world that created Ultron) often worsened situations. She also mentions how her voice still calms him back to himself and how he still has some control over Hulk and that if he ever reached the stage where he almost entirely in control then being able to again won’t be impossible and that she’ll help him with it in whatever way she can. There can be a scene of her giving the vial of the "cure" to Nick Fury to put it in safe storage, in case it ever becomes essential but that she has no desire to use it any time soon. They then sit together arm in arm with Black Widow saying,

"Sun's getting real low."

I'm going less green just imagining her voice, I'll always be slightly green though I have to accept it

Possibly said during a sunset, but this might be too un-subtle a detail.