Signs have been pointing to Black Panther as a prime candidate, along with Dr.Strange, for awhile now.
Marvel also just recently did a Black Panther Essential
. Which for those who don't know is an all b&w reproduction of a characters origin and 25-30 issues of early appearances.
Having some material collected and in an easily accessible and affordable format is one possible sign many point to for BP film.
Look how the Thanos material is red hot now. Marvel dropped the ball by not having current reproductions on the shelf there. Ebay is on fire with the stuff.