Roadmaps Including Both Classic and Next-Gen Projects
To create a roadmap that includes both classic and next-gen projects, you will need to create a board that includes both projects:
Navigate to your classic board > Click on the three dots Icon > Board settings > Edit filter query
Add your Next-gen issues to be returned in the board filter
Back to your board > Project Settings > Columns
Map the statuses of your next-gen issues to the columns of your board
Make sure not to skip the status mapping step. Then when viewing the roadmap for this board make sure that both of the epic types are checked in the options menu to ensure all epics from the board appear for selection.