Lots of cool new unseen Xel'Naga and Hybrid designs are in the new art book from late last year / early this year.
Here's the link:
Also, here's a bonus that seemingly implies Kerrigan's "armor" is actually creep and works like the Venom Symbiote:
I highly recommend getting this art book, it looks amazing, there's a lot more stuff in there (that I'd wish they'd release in HD or 4K PNG quality) that I'd wish they'd use.
From this art book, it seems that they drew quite a bit of inspiration from fans. Looks like they drew inspiration from fan art for some of the Hybrid concepts, too:
We also know they took the whole "Kerrigan gets de-infested and leaves with Raynor" plot line from a few fan campaigns. Duran being a Xel'Naga shapeshifter, as well. But yeah, we already know about these. I wish Blizzard's story team would've surprised us more when it comes to SC2's story instead of catering to expectations and rehashing fan ideas.