As we know in Captain America 3: Civil War, Captain America and Iron man are going head to head with there own teams of heroes fighting against each other in a full out war, and the obvious question would be who (if they could) would ask the Hulk/Dr Banner to be on there side first? and as the Hulk is such a powerful character I'm not sure he will be there like in the comics but it has been stated that This Movie version of Civil War will be different than the Comic version, so here is a theory if the Hulk is appearing in Captain America 3: Civil War where he would be, followed by to theories on where else he could be.
Captain America Civil War: The Hulk goes rogue
The possibility of this is slim but would make a unique dynamic in Captain America 3, if they Gave Hulk the Story line of The Thing in Civil War. In the comics The Thing is a reluctant member at Iron Man's side until the on going war kills a civilian from his Neighbourhood, The Thing shouts both sides for not caring about the civilians caught in the conflict. He announces that, while he thinks the registration is wrong, he is also not going to fight the government and thus leaves the country.
If you didn't know Marvel cannot use The Thing in their universe as his rights, as well as the rest of the fantastic 4, are with FOX, so giving this story to Bruce Banner/Hulk would be very interesting due to the fact of when he becomes the Hulk He worries about hurting civilians
[Guardians of the Galaxy 2](movie:1081113): Planet Hulk
A popular rumour that has been going around ever since Guardians 2 was announced is that the Hulk might appear, since then Marvel has said that's not happening but they might be lying so people don't see it coming.
If you don't now what Planet Hulk is or entails its a story/DVD film where the hulk is sent into space and crashes on a planet and becomes a gladiator and then the king of that planet, which then goes into the story of World War Hulk where he returns to earth with is army to take revenge on the people who sent him there.
I don't know how plausible this is as it would be mostly a hulk film than Guardian of the galaxy 2 but if it is done in a way where you know its happening and get some glimpses at planet hulk I could see world war hulk merging with Infinity war, as in Thanos recruiting Hulk and his army (after credit scene) to invade earth with Thanos which would put the WAR into infinity war.
Thor: Ragnarok (Hulk vs Thor)
[Thor: Ragnarok](movie:956858) the apocalypse one, not the clone.
Now this is a quite interesting concept as Ragnarok is mentioned plenty in the animated film Hulk VS Thor, where Loki and the Enchantress brings the hulk to Asgard separates him from Bruce, uses the hulk as a puppet (literally) and starts destroying Asgard trying to get the Odin a Kill him during the Odin sleep, but Loki Loses control thanks to Thor and now a Hulk with out the conciseness of Bruce to hold him back goes on a rampage killing everyone and is Creating Ragnarok which means the end of days (or something like it).
Now obviously Loki can't do this cause he is acting as Odin
If this was going to Happen I think that it would just be the Enchantress as she is a past lover of Thor and wants revenge on him as payback and it fails but releases Surtur (a fire demon) who Starts Ragnarok so then Thor And Hulk Have to stop Surtur from destroying all Asgardians,