It is a pretty typical situation.
Obviously you can not show the work publicly in a portfolio unless you want to be fired and get a reputation that you can’t be trusted.
Your best bet is to do some concept work that shows the skills you have developed.
Your best bet, if this will be a big product, is to just wait until it comes out. If you leave now, the company will credit someone else with the design. That happens all the time. Once it is public, you will have the most leverage in securing the best position with the best pay.
Is there a reason you need to leave the company now?