Alright, people, let's dig into this whole mess.
Now, Black Panther does have some characters who have appeared in previous MCU movies, like Martin Freeman's CIA agent, Everett Ross, and Andy Serkis's Ulysses Klaue. Also, T'Challa was a huge part of Captain America: Civil War. But it may be following a pattern that has emerged in the second and third phases of the MCU.
If a S.H.I.E.L.D. agent or an Avenger doesn't show up in the movie, an Infinity Stone usually does.
Well, we know that vibranium in its metallic form can be silver. Also, Infinity Stone containers have differed in color from their stones before. The Chitauri scepter was blue until it was cracked open and revealed to be yellow.
The heart-shaped herb checks off a bunch of properties the Soul Stone has in the comics:
• It can trap souls inside of itself in an idyllic pocket dimension, like the Lion King dimension T'Challa goes to during the herb ceremony.
• Its wielder can access the memories and skills of those imprisoned within the pocket dimension, like T'Challa's conversation with his dead father.
• It can also revert beings to their natural state, like removing T'Challa's powers.
The whole thing with Wakanda is that they hid from the world for centuries and developed advanced technologically with the help of vibranium. Whereas Howard Stark and S.H.I.E.L.D. got their hands on the Tesseract and tried to turn it into a weapon and screwed everything up.
Also, why would Black Panther get involved in the fight against Thanos if there wasn't a direct link to Wakanda? Remember, this is a kingdom that was totally cool watching from the sidelines while the Chitauri leveled Manhattan.