This is why I was hoping JR would bring in another top four guy this past summer. Obviously the ship has sailed, but a guy like McDonagh would have been huge. A top pairing guy who can also log 23+ minutes and play in all situations, and who is good enough to carry his own pairing, making the second pairing essentially another top pairing.
Obviously Letang playing like he has this year has just made the issue worse, but I think the rest of our defense are more complimentary players on a pairing rather than guys who can carry or anchor a pairing. And when they're not at their best, those pairings simply fall apart.
I like the Hjalmarsson suggestion a lot, but I'm not sure how reasonable that idea is. Put Hjalmarsson with Letang, Dumoulin with Schultz and Maatta with Oleksiak. Your D pairs are instantly good with that, because you definitely have a guy who can anchor each of those pairs. Dumoulin continues to remind me more and more of Paul Martin, especially with how his offensive game has seemingly progressed this year. I know Dumoulin's point totals aren't any better, but I feel like he's been better offensively this year than in recent years. He won't reach Martin's best, but I think he can reach what Martin was most of the time.
If you don't do anything, the Penguins defense isn't bad. Letang is clearly a #1, even when he's struggling (hot take, I think he has played well in 2018 too). Dumoulin has sold me on him being a #2, and if not a #2, a #3 who can anchor the 2nd pair at worst. Schultz is....I'm not sure. He's been surprisingly effective defensively this year, but his offense is a lot worse. I think he has been more of a 2-way defensemen this year than an OFD, which isn't what you want to see. He should be a #3 OFD, but I see him this year as more of a fringe #3/4 2-way D this year. Maatta is about the same I think, I'd consider him a fringe #3/4 2-way D. I think the Penguins defense would be better off with Maatta-Letang and Dumoulin-Schultz, but I don't think you'll ever see that. The defense isn't going to change at this point.
The Penguins have a #1, a #2, 2 complementary middle pair defensemen, a complementary bottom pair defenseman (Oleksiak) and 2 #6 defensemen (Hunwick and Ruhwedel). They need a top-4 D of some capacity to push everyone below Dumoulin down a spot.