Warning: Spoilers ahead. If you haven't seen the film, do not proceed.
It's been a week since Avengers: Infinity War hit theatres. By the end of the film, you were left wondering WTF just happened. So let's address the elephant in the room.
Banner can't seem to get it up. We saw
him The Hulk at the beginning, taking a beatdown at the hands of Thanos and escaping after being sent to Earth by Heimdall. For the rest of the film, Hulk never surfaces, even when Bruce Banner wants him to.
We have a few possible theories about why that happened and what is next for the green rage monster.
1. For the first time, the Hulk actually is scared of something.
MCU's Hulk had never been defeated prior to Avengers: Infinity War, even if you count The Incredible Hulk as a part of the MCU. He was badly beaten down by Abomination but that only made him angrier and he nearly killed the latter.
Even when Tony Stark used the Hulkbuster on him in Avengers: Age of Ultron, The Hulk was never really in danger. You have to understand that this is a unique creature with the strength of a God and the mind of a child.
A child, who prior to being beaten by Thanos, believed himself to be the strongest creature in existence. He literally spent two years in Sakaar as an unbeaten gladiator. Hell, he even tried taking on a fully powered Surtur in Thor: Ragnarok.
So when he stood up to Thanos and bit the dust with a span of seconds, it is possible that for the first time, he contemplated the possibility of not being immortal, which is why he didn't show up to the party.
2. Bruce Banner is not angry anymore and can't summon him.
The Hulk is an undeniable truth that Banner has been running from, since forever. The Hulk has been treated as an entirely different person in the MCU, who manifests at the most unfortunate moments for Banner.
In The Avengers, we saw that Banner had somehow managed to control The Hulk breakout. In Avengers: Age of Ultron, he willingly gives the control to The Hulk only to be woken up by Natasha Romanoff.
Following the events of this film, Hulk decides to leave Earth where he is tired of everyone being scared of him. In Thor: Ragnarok, we find out that The Hulk feels lonely like the rest of us and likes being admired and cheered for. And hence, has been in the driver's seat for 2 years, living the life of a gladiator.
At the end of the film, however, The Hulk refuses to surface when Banner jumps from a Helicarrier, indicating that the Hulk has asserted some control over his summoning.
This could be because Banner has finally made peace with the monster within him and had very little control over their shared body due to not being angry anymore.
3. The emergence of Doc Green, a Hulk with Bruce Banner's intellect.
Doc Green is a version of the Hulk that has the intellect of Dr. Banner and the strength of the Hulk but does not identify himself as either of them.
In the comics, when Bruce Banner was repeatedly shot in the head by an unknown assassin, Tony Stark injects him with the Extremis virus. Remeber Iron Man 3 and all those burning bad guys?
So yeah, Tony injects Banner with the same virus making him turn green. Except, this time, a different version of the Hulk walks out. When people call him The Hulk, he refuses to acknowledge the name, saying that it was given to him by his captor.
It's highly unlikely that the MCU will stick to the same origin story but it's possible that Doc Green could be given the green light in the future films. This could very well be part of the Hulk's character development and storyline that has been running through the course of the MCU.
4. Maybe The Hulk finally witnessed what Scarlett Witch made him see in Avengers: Age of Ultron.
We are yet to find out what Wanda Maximoff made Banner see that turned him mad enough to become the Hulk and destroy half a city.
But we also know that whatever she made the others see, it all came true (the past in the case of Captain America and Black Widow). Thor saw himself being responsible for the death of Asgard and Tony Stark saw all his friends die.
It is quite possible that the Hulk witnessed something during the events of Infinity War that Scarlett Witch had already made him see, making him go into a shell of his own and never coming back.
5. We might finally get to see The Hulk that gets bigger and stronger as he gets angrier.
In the comics, one of the many powers of the Hulk is adapting to his environment. He has superhuman speed, can breathe underwater and even survive in space.
The gamma radiation in his blood manipulates his genes to mutate according to the threat he faces. So the Hulk increases in size and strength as he gets angrier. Although his size is not proportional to his strength.
Meaning, while the strongest of the Hulks (The Savage Hulk) have been known to be around 8ft tall, their strength increases vastly. Hopefully, we get to see some of that madder more badass Hulk in Avengers 4, when he takes on Thanos.
So, that's all the plausible theories we could manage to find. Of course, there are many more floating around but MCU won't probably touch them with a 10-feet pole.