Regarding "where is the last infinity stone?," I don't consider this to be a spoiler since this only reveals what we DON'T know as of Ragnarok. But I include this little preamble for those who don't wanna know so you can stop reading.
Okay, so we know it isn't in Ragnarok. (I haven't seen it, but I take others' word for it) I just read this on Screenrant:
Thor: Ragnarok is the 17th feature film by Marvel Studios, leaving five other movies still to come before the party wraps up. There are therefore a options for where the Soul Stone may be:
1.•Black Panther – February 16, 2018
2.•Avengers: Infinity War – May 4, 2018
3.•Ant-Man and the Wasp – July 6, 2018
4.•Captain Marvel – March 8, 2019
5.•Untitled Avengers 4 (Its title is a minor spoiler) – May 3, 2019
However, given the nature of Avengers: Infinity War, it’s a certainty that the Infinity Gauntlet will be used in this film next year by Thanos. That means the Soul Stone must be introduced in the next few months. It would be weird and almost cheap from a storytelling standpoint for it to just briefly get introduced in Avengers 3, so it’s a safe assumption Black Panther is where the last piece of the puzzle remains.
So, throw those Thor: Ragnarok theories about Hela (Cate Blanchett) having the Soul Stone, or it being in Odin’s Eye, or Heimdall having it as his own source of power, in the trash. The Soul Stone is in Wakanda, the secretive, isolated fictional nation in Africa where Black Panther reigns, and it makes a lot of sense when you break it down.
I STRONGLY disagree with the bolded. Yeah, it
could be tied to Black Panther. But there is no reason that has been revealed thus far that it
must be. And, honestly, I don't think it is. I think it would be silly for it to be there. Contrary to what the article says, I think it will ultimately be first revealed in Avengers 3. It makes sense to me that it will be someplace outside of earth that has not been revealed yet. And I think Captain Marvel is the perfect vehicle for that. I'm thinking Avengers 3 is mostly about that stone. And then after that film, we get the Captain Marvel film, which is set in the '90s, and get some back story on it. And then we get the conclusion in Avengers 4. That makes the most sense to me. I guess we'll see. But that is just a common sense way to do it, based on what we know is in store for the next few films.
I think this next bit is more insightful:
On the other hand, why are so many of these Infinity Stones finding their way to Earth? What’s so special about our planet in a very populated galaxy?
That is, in a nutshell, why it would be dumb for it to be on earth. With so many infinity stones on earth, how could it be that nobody in the MCU outside of earth wouldn't have made a major play to wipe out earth and obtain them much earlier in the planet's history (i.e., prior to the existence of the Avengers) when we would have been FAR less prepared to deal with such a threat? It just doesn't make any sense.