Some organizations formally track risks and opportunities in projects.
Although there are dedicated risk and opportunity software tools for sale, our preference is to put actions to mitigate risks and to capture opportunities as task level work item cards directly onto the Kanban board.
You can put due dates on the cards. Some tools like Trello also provide a calendar view. You may want to use a special color for these tasks or just use yellow sticky notes for all tasks including this type. Some virtual board services allow labels that let you filter cards based on their label. This becomes more important when you have a few hundred work item cards on your board. Not only does this provide a way to track these important tasks to closure, but it also provides additional historical data that can help estimate future projects.