I think we saw either the Soul Stone or the Mind Stone in Doctor Strange.
Not only that, I believe the ending to Avengers: Infinity War was also teased.
The key scenes take place while the Ancient One is showing Strange the Multiverse.
At one point, we see Strange get multiplied into six bodies. Each of the copies are more disintegrated versions of Strange. I think this could have been foreshadowing that Strange would be turned to dust at the end of Avengers: Infinity War.
Next, is what I think is the Mind Stone or the Soul Stone.
Immediately after the previously mentioned scene, Strange is sucked into something like a black hole or a whirlpool.
I should also note there is more dust looking stuff all around Strange that also gets sucked into the black hole. This could be referencing everyone else turned to dust by Thanos.
Now, in the center of the black hole/whirlpool, there is a large yellowish/orangish glowing circle.
I thought this could have been the Soul Stone because Strange and presumably other people were being sucked into it.
However, it could also be the Mind Stone as in Avengers: Age of Ultron, after Thor brought Vision to life, he mentioned he had a vision of a whirlpool that sucked in all hope of life which had the Mind Stone in the center.