AVENGERS: AGE OF ULTRON SPOILER WARNING!!!
If you haven't seen Avengers: Age of Ultron then why on earth are you reading an article about MCU continuity? However I will mostly be discussing "moderate" spoilers as any good researcher could have come across [most of] these facts on the internet even before the film premiered, but to be on the absolute safe side: Go Watch The Movie.
You can't say I didn't warn you...
What We Know So Far:
Okay... now that we have that out of the way, let's talk about bridging Avengers: Age of Ultron with Captain America: Civil War, since the MCU has created the new "Cinematic Continuity" genre. First we know from Marvel's Fall 2014 Announcement that Black Panther/T'Challa , played by Chadwick Boseman, will appear in CW, and several connections to T'Challa's home nation of "Wakanda" were established in [The Avengers: Age Of Ultron](tag:293035) First the Avengers travel to Wakanda, "meet" Ulysses Klaue (Andy Serkis, who gets his arm chopped off) and then fights Ultron and the Twins, followed by Scarlet Witch hexing Dr. Banner, forcing the Hulk to go into a rampage through the nearby Wakandan city. Klaue is Panther's main villain, and T'Challa is the prince/king of Wakanda (depending on when we first see him). After finally dispatching the Hulk (ohh, that was such an awesome fight, finally a true Hulk showcase. Thank you, Marvel), and the team "flees" Wakanda in the Quinjet, Tony Stark asks Maria Hill if the Stark Relief Foundation had been contacted, to which Hill replies that they were already on site.
We also know that Marvel was undergoing negotiations with Sony Pictures to share Spider-Man, and that while they were developing the story for CA: CW, Marvel created two scripts: 1) No Spider-Man, and Black Panther would be the pivotal "Middle Man" role between the Stark-Rogers fight, which Spidey had fulfilled in the Original Comic Story (OCS). 2) Spider-Man is present, and plays a considerable role in the film's story, with Panther still being present, but in a smaller role. Finally it was announced this year that Sony and Marvel had done the unthinkable: made a joint Spider-Man canon deal :D
How This Can Lead to Civil War:
Now that we've established the facts, let's get to the theorizing. The OCS for CW launches the Superhero Registration Act debate via a supervillain catastrophe, which killed many innocent civilians, causing the general public to demand such legislation from the U.S. Congress. In A: AoU Hulk's rampage in the Wakandan city more than accurately fits the "adaptation bill" for such a catastrophe, especially since the Hulk is supposed to be a superhero, therefore it would be logical for the public to demand government-level oversight and accountability in response to a superhero catastrophe, instead of the OCS incident. Dr. Banner himself pointed out that "the world has finally seen the Hulk, the true Hulk", giving credence to the level of destruction that the tiny nation had endured by his hand. Although the people of Wakanda certainly aren't Americans, and wouldn't really have any right to "petition" the U.S. Congress to do anything, but since most of the Avengers are American (save for the Scarlet Witch, and Vision who has no citizenship, with Black Widow most likely having been Naturalized), and SHIELD supporting/funding the team, Congress is really the only authority that the Wakandans can go to "put a leash" on superhero activity (namely, the Hulk).
The Wakandans may indeed blame Hulk, but since Banner has gone into deep hiding that neither SHIELD, nor even his best friends, can find him (and you thought Bucky was hard to find), the Wakandans don't directly demand for his arrest (which was teased in A: AoU), but speak in the "general terms" of "superhero oversight", with Hulk being the literal "elephant in the room". Some members of Congress/US Government, may feel that if they offer Banner up as a scapegoat to the Wakandans, their own rage will be at least lessened, if not quenched, so he/she/they will call in the best Hulk-Hunter on the planet: General Thaddeus "Thunderbolt" Ross (whom Marvel has confirmed will be reappearing in CA: CW). The Wakandans may also see the Stark Relief Foundation the same way that the people of Sokovia saw the Iron Legionnaires (throwing rocks at them) as an extension of the Avengers' unchallenged "dominance" over the global populace, and begin to attack the very relief workers that are trying to help them rebuild. T'Challa, as the prince/king of Wakanda, could side with his people, and request/testify before Congress that action be taken (thereby allying with Stark) or he does hear the cries of his people, but understands the trials and struggles of the heroes (thereby allying with Rogers). The Hulk rampage (since Wakanda is a "micro-nation") could have decimated Wakanda's economy/industry/infrastructure, convincing T'Challa that the U.S. Government must pay restitution for the damages rendered; yet considering his run-ins with Klaue, T'Challa may be mistrusting of all "outsiders", and thereby look at Stark/SRF as "invaders" into his country. Spider-Man, in the OCS, "ping-ponged" back and forth between Camp Stark, and Camp Rogers, so if Black Panther was originally going to take this role in the first script, these elements might come into play with him allying alongside one party, and then switching sides later, even though his overall role in the film will be lessened than originally envisioned.
Now I know that A: AoU's script was developed a whole year before Marvel began talks with Sony, and therefore too much of Wakanda was already established and solidified in the film by the time the negotiations took place, that re-shoots were impossible. That said, I highly doubt that Marvel would let all that effort of putting all this knowledge about the small fictitious African nation to waste, and considering the fact that Marvel has since pushed Black Panther's solo film back to 3 years from now (instead of moving Doctor Strange, Guardians of the Galaxy 2, or even just shifting the Infinity Wars films each back a year) which is a REALLY long time to try to keep less major facts about a film in the public's memory, so they're going to HAVE to use something of Wakanda, and a celebrated actor like Chadwick Boseman, in [Captain America: Civil War](tag:994409).
After all, this is only a theory, and I'm focusing on Wakanda/T'Challa's involvement. I'm not going to theorize about who picks which side, outside of what I've written above. Let me know what you guys think of my theory(s). But please don't mention any "leaked" script, I already checked to see what Marvel has officially announced, when I found reference to alleged "script". It's been two weeks since that thing was released, and Marvel has taken no moves to bring it off the Web, or discredit it. And for anyone out there who might still be hopeful that's a genuine script: there is no mention of Paul Rudd's Ant-Man anywhere in the script, whom both Marvel has confirmed will be appearing (see here) and Rudd was photographed on the ongoing set of CA: CW.