Natasha, Pierre, & The Great Comet of 1812 is pretty sophisticated. It's a completely sung through opera made up of mostly recitative, with an orchestra made up of a mixture of traditional orchestra, Russian folk band, rock n roll band, and a DJ... it's pretty cool.
THE CRUCIBLE, by Robert Ward... never played broadway but it debuted at the Met and won the Pulitzer for music. It's one of my favorites and it's worth watching. It's one of the scariest/coolest scores you'll ever hear.
This seems obvious but most of Sondheim's stuff is pretty complex. Sunday in the Park with George, Passion, A Little Night Music, Assassins, Follies, and Sweeney Todd are probably the most complex of his scores, with Night Music, Passion, and Sweeney performed pretty often at Opera companies across the world.
Also look into Kurt Weill. There are great recordings of THE RISE & FALL OF THE CITY OF MAHAGONY (with audra and Patti Lupone) and STREET SCENE (which played Broadway).
Also check out Michael John LaChiusa's stuff. All of his stuff is super complex, lush, and gorgeous. GIANT especially, FIRST DAUGHTER SUITE and QUEEN OF THE MIST which just came out... and basically everything of his.
Definitely check out I AM HARVEY MILK by Andrew Lippa... you're going to freaking LOVE IT. It's the best thing he's ever written.
Also look into:
THE MYSTERY OF EDWIN DROOD by Rupert Holmes
SUSANNAH by Carlisle Flloyd
RAGTIME by Ahrens and Flaherty
MISS SAIGON (make sure to get the original cast recording though) by Boubil & Schönberg
PORGY & BESS by Gershwin
CAROUSEL by Rodgers and Hammerstein
FUN HOME by Jeanine Tesori
THE BRIDGES OF MADISON COUNTY by Jason Robert Brown (esp if you love Parade & piazza)
TITANIC by Maury Yeston
I hope that helps. If I think of any more I'll edit this post. :)