I'm not sure Denzel would take a supporting role a film in which he probably dies within the first half an hour. Also Denzel's salary is probably outside of Marvels usual pay structure. Denzel would have to take a pay cut and probably be really enthusiastic about the film to do it.
I think it might be interesting if Marvel asked Denzel or Forest Whitaker to direct Black Panther. Both have directed movies before and are veterans in the industry so know alot about filmmaking.
If Marvel want a name actor as T'Chaka then I think Laurence Fishburne, Danny Glover, or Wesley Snipes might be a choice.
If they want a popular veteran black actor who isn't necessarily a household name then Dennis Haysbert, Delroy Lindo, Harry Lennix, David Oyelowo, David Harewood, Paterson Joseph, Eamonn Walker and Hugh Quarshie are all great choices.
Eamonn Walker in particular I think would make a great T'Chaka