You can put anything you want to in the middle but some traditional items would be: korean radish, carrot, egg, spam (because it IS an essential meat in SK), cucumber, garlic, peppers, ground fish patty, spinach...and the list goes on! I have literally had various locals stuff their personal kimbap in my mouth because they want me to "oooo" and "ahhh" and their selection of foods inside.
The potential spread: before
I wanted to make kimbap and let my Korean friends tell me how they would alter it to make it better. I didn't get that far though because it popped on me....
I'll keep working and I'm sure by June I'll have it down!