I just finished the first mistborn series and loved it! I didn’t know about the Cosmere before I started but now that I know, what should I read next? I’m leaning toward the next mistborn trilogy.
If you loved the first Mistborn trilogy, you can't really go wrong with any of Sanderson's other works, really. A logical step would be the second Mistborn trilogy, which can stand on its own, but you'll definitely enjoy having just read the first one, having that universe fresh in your mind. But if you don't feel like hoping into another large undertaking, there's lots to read, such as The Emperor's Soul, the Legion novellas, the Reckoners (which is also a series, but perhaps a bit lighter in terms of concepts, very action-oriented. Then, of course, there's the Stormlight Archives, his most extensive work, which is truly epic and definitely worth the read. Plenty of other stuff still, the man is a writing machine! Enjoy discovering these, I wish I could read them all again for the first time!
Secret History is a great read to revisit what happened during the original mistborn series from a "secret" perspective.
If you want a break before diving into another trilogy or series you can always check out Elantris , Warbreaker, or The Emperor's Soul. They're all stand alones that take place in the cosmere and are quick reads.
But as someone re-reading Mistborn, I love the Wax and Wayne series, they're great.
As others have said, you can jump to any other cosmere series and you'll love them too.
I personally would put Era 2 on the back burner for now. It includes much more Cosmere info than Era 1, and you'll miss out/not understand a lot of it, if you don't read some others first. My suggestion would be doing Warbreaker or Elantris next. They are both standalone books, it doesn't matter the order you read them in, but Elantris was Sanderson's first published book.
From there, I'd do the first 2 Stormlight books followed by Arcanum Unbounded. Arcanum is a collection of short stories, all Cosmere. It includes Edgedancer (Stormlight 2.5) and Mistborn: Secret History. The events in Secret History happen before Era 2, so would be good to read that right before getting into Era 2.
After Arcanum, you can choose to continue with Stormlight, or go back to Mistborn. It's a fairly good pausing point for the Stormlight series. There's also the White Sands graphic novels, and 3 standalone books released this year (Secret Projects), that you could sprinkle in between some of the thicker novels.
The next set of the Mistborn saga would be a natural choice. You may need a period of adjustment for the first couple of chapters--it's centuries later than the first trilogy--but I think you'll really enjoy it! Wax and Wayne are great characters.