I have a grid list made of cards. Each one of this cards contains 4 list item. If the user tap on the list item, the card is expanded in order to become a hero and to show the contents related to that list item. The icon must convey that the element is an interaction element, but it should also communicate that it is not simply a link or an accordion.
I'd suggest trying to find an UI pattern that fits this purpose - so users are familiar with it, and will find it intuitive to click there to 'expand' and have more content.
In your case, perhaps 'card expand' according to Google guidelines (Material Design? is a possibility?
Or even the known '+' icon if it suits the functionality better. https://material.google.com/motion/material-motion.html#