I am currently reading Anna Karenina by Leo Tolstoy and think it's a good candidate for this list because of how much fun I am having reading it, and also its many explicit themes (outlined below). It's a whopping 750 pages (by the Barnes & Noble classics version's page count) but is quite compelling and is an interesting, historical read. It's about a woman named Anna Karenina who is married to an upstanding and pious member of the Russian government and has an affair with a young cavalry officer which brings brings about a change in her social standing and makes her an outcast of society. Simultaneously, the story of a young landowner named Levin is told through alternating chapters. Levin is seeking his own fulfillment by pursuing the love of Anna's sister-in-law's sister, Kitty, but through more socially acceptable, righteous means than Anna. There are a lot of themes present in the book that would make for great essays, including religion, adultery, and an exploration of class differences. The book also has great potential for a compare/contrast piece.
I love this novel, and would recommend it to anyone who's willing to put the effort in and read all 750 pages-- it's surprisingly easy to comprehend the language, and my version has plenty of helpful footnotes explaining anything the reader might not be familiar with.
I'm also a big fan of Mansfield Park by Jane Austen, as well as Sense & Sensibility, and I know for a fact that Mansfield was listed among the possible selections for our independent book of choice projects, so keep that in mind too. Wuthering Heights by Emily Brontë was super entertaining, though I'm not sure it has quite the same essay potential as Jane Eyre did; I don't seem to recall any major messages in that book besides angst and romance, but it is of literary merit.
I'm sure there's a dozen more that I'm forgetting but that's what's in my head right now that I think may not be obvious choices and may interest people. Looking forward to future suggestions!