A 6'9'' 230 lb center who averages 15 rebounds a game without great length or athleticism ... not sure you're gonna see that in this league anytime soon.
Cowens was a great player from a very, very different era.
David Lee is the best comparable.
That was the guy who came to mind for me, too.
Again, Cowens was a great player. I'm not trying to discredit guys based on the era they played in. I just don't think you're gonna see another guy who plays that way be dominant in that way.
That said, I think it's generally agreed that the NBA was kinda watered down during a significant chunk of the decade in which Cowens played, though, due to drugs and the ABA taking some of the talent.
Kevin Love might be the closest thing, actually, given his dominance of the boards despite not having great size or athleticism. But Love is an inch taller and at least 20-30 pounds heavier. Plus he's got that outside game, and he's very clearly not a center. Cowens was a far, far better defender.