rpmbuild: Is there a way to specify recommended packages

Yes, it is possible, but not on all distributions.

• Fedora>=24: Weak Dependencies are Suggests & Recommends and Supplements & Enhances
• SUSE>=10: Suggests and Recommends

Haven't found references for CentOS and others yet.

RPM has no concept of "suggested" packages, like DEB has. I've been involved in discussions about this, and the conclusion was that it just doesn't make much sense. "Suggested" for what use? "Suggested" can go from "You might want to look into this too" to "Very unlikely to be of any use without..." Why would you want to need the suggestion? Either it is required, or the user knows what to do. RPM is for fully automated setup, stopping to ask the user if suggested packages should be installed just goes against its basic philosophy.