Blogs with the proclamation that DC/Marvel has big plans for blank character is nothing new. But yeah man, any day now Duke Thomas and Harper Row are gonna be a big deal. A BIG deal.
Duke Thomas is still too fresh, it's too early to tell. He is Scott Snyder's character and All-Star Batman is featuring him prominently - but so far he is a flat character with an uninteresting back story and a looking costume. Rumour is he will have his own book after All-Star wraps up but I don't see it happening unless he breaks out in the latter half of All-Star. The Bat Family is getting bloated and we don't need another Bruce Wayne cast off.
Damien Wayne on the other hand is established. He featured in the DC animated universe already as of 2014, has a famous mother / family background, has his own team book in Teen Titans in Rebirth, and is going to be part of the young Trinity next year in Super Sons. The potential for Damien to be a modern DC staple character is already very high.