Substitute for shrimp in okonomiyaki

Haven eaten okonomiyaki at a famous establishment for it in Akasakusa Tokyo, its the equivalent of a Japanese Omelette-the ingredients are varied, and can be combined at will, based on preference. This particular establishment provides you with your ingredients, that you choose, and then you actually make it yourself on a flat griddle that you are seated at.
Basically, anything goes!

I grew up eating it with chicken and steak. You really can do any protein you prefer or none at all.

You can just leave it out which would make the okonomiyaki more in the Osaka-style.