Hello @Dan Cumings
Thank you for reaching out.
The roadmap functionality of Jira Next-gen projects is based on the board filter, just like all the other board features.
That being said, I'm afraid the board filter for Next-gen projects cannot be edited at this time. In fact, the Next-gen template was designed to be a simpler and straight forward solution for those customers who felt overwhelmed by the complexity of Jira application, however, we agree that this simplicity can be improved if we add some optional features to it.
With that in mind, we created the following feature request to add the ability you are looking for:
Feel free to vote and watch the suggestion to increase its priority and also receive notifications about any updates.
Let us know if you have any questions.