There is no requirement in ISO 14001:2015 to incorporate the opportunities in an Environmental Aspect Impact Register.
Different organizations use different approaches to manage risks and opportunities and aspects and impacts.
For example, when I audit organizations sometimes, I see this approach:
When I work as a consultant, I use this approach:
According to the first approach, opportunities are significant positive environmental impacts. According to my approach I have two registers: one for risks and opportunities and other for environmental aspects and impacts.
The following material will provide you more information about aspects and impacts and risks and opportunities: