So I found this website by one of the guys who did concept art for Knight's Kingdom (incidentally, he also did some concept art for LU too). He has some really cool art there, as well as some behind-the-scenes descriptions of the development process behind Knight's Kingdom. Here's the link to his site if you want to check it out.
http://www.mikerayhawk.com/knights.htm
Also, there's this site BrickLink, on which you can buy individual lego bricks and stuff, which has pictures for a bunch of the old Knight's Kingdom trading cards on its site. I don't know if it has all the cards, but it has a lot. The cards have a lot of interesting pieces of Knights Kingdom lore. Here's the link to that:
https://www.bricklink.com/v2/search.page?q=knights%20kingdom%20card#T=G&P=1
As you probably already know, I tend to have an "include everything from Knight's Kingdom in KotOS" mindset, and I hope that all this cool Knight's Kingdom art and lore wins you over to my point of view. :P