Anyone who isn't Wakandan, no matter how virtuous they are, won't be able to consume the Heart-shaped Herb without suffering from severe side effects..
On the flip side, Nakia's emotional connection to T'Challa may serve as a loophole to the conditions of consuming the Herb; Okoye is patriotic & would do literally anything for her country, but I doubt she'd be worthy.. Shuri is literally related to T'Challa, so of course she'd be first in line for the throne after her brother's unfortunate passing..