Recently there were rumors about two classic Avengers-members joining to the blockbuster superhero-team in a future movies: Scarlet Witch and Quicksilver. First Joss Whedon hinted that he was adding a brother/sister characters to the next Avengers film; the fans immediately started to speculate that he meant the most famous sibling duo from Avangers comics, and later Entertainment Weekly comfirmed in their article that Pietro and Wanda Maximoff are indeed coming to the MCU, even thou the EW's source is still under doubt by some people. I think this is great move from Marvel and Whedon. Many have been wondering, since the rights to the all Marvel's mutant characters rest with Fox, how is this possible; I will explain that in this article and I will also do some speculation about the characters big screen versions and that what does this mean to the MCU.
The Character rights
Many fans have been scratching their heads, since they thought that because the Quicksilver and Scarlet Witch are mutants, aren't they part of Fox's property, well apparently not completley. Because the characters are such long time iconic members of Avengers from comics, the Marvel studios have equal right with Fox to use them. Same case was with Skrulls, both could use them, but in case of QS and SW, Marvel studios can't have any kind of reference to the mutants, and of course if the characters appear on the Fox's X-Men franchise; they can't have any reference to the Avengers or MCU.
When God(of nerds) takes a mutant, he gives an Inhuman
If Quicksilver and Scarlet Witch are not mutants than what is their origin story, many fans have speculated that they would part of Inhumans. Making them Inhumans would bee the logical choices, because that way the characters could basically be the same. Inhumans are also close to the charcters in comics, especially to Quicksilver, who was married to the Inhuman heroine Crystal, whom with he had a daughter named Luna. Making the characters Inhumans could also be link to the possible Inhumans-movie in the future, which could then be link to the upcoming Guardians Of the Galaxy-movie, thanks to it's cosmic nature. From Inhumans also comes the god-like character Beyonder, this could be chance for Secret Wars-storyline to appear in the future films, even thou it is unlikely since the absence of Doctor Doom.
Daddy issues
In comics, it's not the least part of the twins reputation that they are children of X-Men's archenemy Magneto. Since Magneto is, with out a doubt, a part of X-Men franchise, he won't be appearing in MCU. But because the big-bad-father-character is part of their mytology, could there be an Inhuman counterpart for Magneto; there is the Inhuman Maximus the Mad, brother of Black Bolt, the king of Inhumans. Maximus has tried to overthrow his brother from the throne more than once, Maximus may not have magnetic powers, but he does have a mind-control ablity, could he step into purple-shoes of Magneto? If Maximus felt to minor for replacement of Magneto, than how about the Black Bolt's cousin: The Unspoken, this guy use to be the king of Inhumans until the rest of the Royal Family overthrew him, because of his power hungry ways. He has the ability alter his body into any form he wanted to, and his doings were so cruel that saying his name is forbidden so his just known as "The Unspoken". This evil father character could also have connection to the Thanos, like Loki and The Other had, could be one of Thanos' minions.
Suffering from the case of M-Days
Even thou M-Day was an event that had mostly do with mutants, could there be something similiar in MCU. Knowing the Worlds Security Council's way of doing politics, they might bite more than they can chew with Scarlet Withc, causing her to put things upside down... no, that's a bad idea for a story. I hope there's nothing like that.
Avengers 2, too many avengers?
With all the new characters poping up in the Marvel Cinematic Universe, like Falcon, WarMachine/Iron Patriot, Ant-Man, The Winter Soldier, QS&SW; could the Avengers 2 get overcrowded with characters, I think not. First of all, they don't have to use all the characters they have introduced in the solo-movies, like they didn't have WarMachine in the Avengers. But I got this idea for a plot to the Avengers sequel, that would allow to use more characters: In the begin of the movie a villain(s) start to take down the Avengers team one by one, this forces Nick Fury to start looking for new heroes when the enemy/enemies start attacking. Fury would then recruit heroes like WarMachine/Iron Patriot, Falcon, even Ant-Man could make his debut before having his solo-film later at the same year, that could be good promotion for Ant-Man, since he isn't the most known hero by public. Pietro and Wanda would then show up and offer help since they maybe know villain/ one of the villains. These way the new characters would get more reputation, and the old team could join them for ending battle one by one.
What about the X-Men?
What if the X-men farchise wants to also use Quicksilver and Scarlet Witchs, would that be possible; I don't know, some people think that Quicksilver already made his debut in X-Men Origins:Wolverine among the prisoners that Logan released from Stryker's lab. Is it a first serve with the character rights, or can both Marvel studios and Fox use characters despite what the other is doing with them?
What's your opinion about Scarlet Witch and Quicksilver joining the Avengers?