You can currently add a dependency between features and also between releases. It seems logical that if two dependent features exist in separate releases then the releases are by definition dependant on one another. It would be nice to see this on the release roadmap visualized even when the releases are collapsed and you cannot see the dependency between the features. Otherwise, you are required to maintain the dependency at both levels which can be difficult to keep in sync.
Thank you for your idea. This is not currently planned on our roadmap but we will continue to monitor customer feedback.