It's not anything like Pandora Hearts, but the first thing I'll recommend will be my favorite Urasawa Naoki. He's genius. The maitre. His works have been able to touch the very strings of my soul.

"Monster" is no doubt Urasawa's most famous work, but it's not my favorite one. I think Urasawa starts using his trademark twists in this one, but he has yet to master them like he did in "20th Century Boys". But I understand why people like it. There are a few characters I feel strongly for. I just think the villain isn't quite there compared to his later works.

"20th Century Boys" was the first one I've experienced and oh dear. I can't love it enough. A saga, an incredible journey with the flavor of Steven King's "It" and 70s Japan. It's hard to describe it, when I say it's a journey, I mean it. But the premise is that Kenji, just a guy in his 30s who's working in a supermarket and has a niece to look after, suddenly finds out that a weird cult is enacting crimes they've fantasized with his friends in childhood when they were picturing themselves the world's heroes.
The characters are just so humane, UGH.

"Pluto" is the one that got attention recently thanks to the anime. Though I'd say the manga mostly hits harder except for the first episode (that was a GOOD one). A remake of one of Astro Boy's arcs made into a more "adult" story by Urasawa, it's a touching commentary on the nature of humans (through the robot prism) and powerful anti-war commentary. Sadly, that still remains an eternal theme.

"Billy Bat" is my second favorite. Damn. This plot. I don't know what giant brain you have to have to come up with it. Every history conspiracy woven into one story. Also Disney controls human history @ the manga.

The current ongoing is "Asadora", about a girl who flies a plane. It's great so far! Asa is such a real and relatable girl.

Okay, now to recs that are more like Pandora... It always felt to me like "D. Gray Man" is close in spirit, but I can't explain it. It's a fun read, but now it's in eternal hiatus and chapters update once in two years, so be aware of that.

Be sure to check out "Tsubasa Chronicles" and "XXX Holic"! Both are legends by CLAMP, and I think Pandora was probably inspired by them among other things.

For the Gothic aesthetic, I'd also recommend the "Trinity Blood" manga. The original is a light novel (and sadly the writer passed away without finishing it), but I think the manga is it's best adaptation. I loved reading it back in the days.

I can go at this for a long time. Recently, I've read Dorohedoro, and I loved it. Peak.

Also, "Land of Lustrous" just ended and it's a masterpiece! But I'm sure you've already read it if you're the person I'm thinking about, wink-wink.

My own plans are "Witch Hat Atelier", "Onna no Sono no Hoshi", "The Summer Hikaru Died". I've been hearing from various people they're really good.