Apparently the fourth movie will be called Avengers: Infinity Gauntlet, Zoe Saldana kind of let that slip in an interview a few months ago, but it might not be 100% accurate. Also, as far as I know, those two movies only count as one contract-wise, most likely because they are being filmed together, so we might get another one with Cap at the very least, unless he renews his contract. I definitely think there will be at least a couple of deaths in those movies, otherwise they will pretty much say fuck you to having stakes, since there have really been a single dead Avenger in over 15 movies (poor Quicksilver), and my current guess is Thor (it might even happen on Ragnarök) since he's the most boring of the Big Three. To be honest, I think they have a lot of great things going for them that can become huge and render the original Avengers unnecessary. I, for one, am a lot more interested in sequels to Spider-Man, Doctor Strange, Ant-Man and Guardians of the Galaxy than yet another Cap'n or Iron Man movie. I like how they handled Stark by finishing his trilogy and having him as a supporting character in other movies, he worked quite well on Homecoming. I also don't think the Thor franchise is long for this world, and it's a bummer Natalie Portman doesn't want to be a part of it anymore because she could be amazing as female Thor. Lastly, money-wise, these franchises tend to start a little "smaller" and grow bigger with each new installment, so it's not like focusing on growing these more varied new characters will cost Marvel a ton of money. If anything, it's in-keeping with their tendency to take big "risks" and run with them, even if I'm disgusted at myself and the world by talking about diversity as a risk.