To you OP, not all books/stories are writing about some big name hero. A lot of stories focus on commoners. You don’t have to be a big name to tell a good story.
Recently this is what was done with “the Mandalorian” they pulled out blank slate character nobody knew about before and wrote a story about him and it turned out to be a great success. He is already considered superior to any of his predecessor.
So it’s not necessary a bad move to give a spotlight to a nobody, especially when casual fans or new readers will have completely different outlook on a lot of things about them and they don’t have to do massive research on the character on what he was doing in the past, exploring relatives and such. This is why I see they chose him over Rokhan (even though Rokhan would be imo amazing choice and I’d love to read more about him and especially see more of his interactions with Talanji) and Zekhan deserves a bigger exploration while Rokhan already has a very good relation with Talanji, so he won’t look at her from a more neutral point of view and lastly Rokhan was just promoted to be tribe Chieftain so I guess he can’t work as bodyguard yet again.
Besides a young and inexperienced person will also avoid the traps of a “high ranked veteran not knowing about something he should know or be accustomed to certain things” He offers completely new start, new perspective where already established heroes doesn’t need to show off and since this is his true start as a hero journey his failures and mistakes would be more forgiveable.
And why Zekhan over Thura, Jorin? They’re not Darkspears, that’s why. It would be odd for orcs to throw their lives for the loa they don’t really care about. The quest is to secure Bwonsamdi. And to continue work on bond between Zandalari and the Darkspears and only the latter are the only Horde’s subfaction that Talanji actually cared and admired. Having any other race member in place of a troll wouldn’t really do it imo. Bwonsamdi bonds Zandalari and Darkspears on a religious aspect.
However I agree that orcs also needs their roster expanded. Still Darkspears were long overdue for a new champion.
If Zekhan has to get out of Saurfang’s shadow and if we’re meant to learn that he is much more than that - then a book is the best option to give him exactly that. I highly doubt that by the end of the book he will be next World Shaman, or some OP being. But I do expect he will be slightly more accomplished Darkspear shaman, a proud champion of the tribe and a truly man on his own and not a kid that covers behind bigger figures.
To make it short: Fresh new characters being the focus of a story is not a bad move at all, it all depends how it will be executed.