From what I recall, T'Challa was meant to have a smaller role at first. Because they wanted Cap's side, Iron Man's side and the third/neutral side that they wanted for Spider-Man.
But the Sony negotiations hit a rough patch and was looking bleak, so they decided to write that 3rd role perspective to Black Panther. Prior to that he still would've been in the movie, but more like a cameo as opposed to a whole arc. It just so happened that after they already went with that decision, the Sony deal went through and so they plugged Spider-Man in as well.