Speculative theory: Ultron looking for that Vibranium upgrade brings the war to Wakanda where the Avengers follow. Death, chaos, destruction..... T'Chaka falls in combat saving an Avenger from a death blow delivered by Ultron.
T'Challa is set to take over as king of Wakanda, Nick Fury offers him a spot on the Avengers just as was previously offered (a position in SHIELD) but declined by his father many many years ago. The offer is neither accepted nor declined as pressing Wakanda business and tradition for inducting a new King from the line of heirs takes precedence to all other affairs.
Post credits scene can be T'Challa being notified that he's receiving an encrypted call from Nick Fury right after his crowning ceremony has concluded so he enters the private control room chamber behind the throne, where Fury debriefs him, reminds of the earlier offer and warns him of a threat in the area. T'Challa acknowledges and ends the call, then we see him accepting the mantle of The Black Panther. Credits roll.
This leads to the Black Panther solo movie and eventual inclusion in Avengers 3