Is there some standard protocol to suggesting reviewers/referees for a paper?
No, the editorial board serves a different purpose from the reviewers. Typically, your paper will be assigned to one member of the editorial board, who will be responsible for managing the review process, and making a decision based on the reviewers' recommendations. However, the reviewers may be anyone with sufficient knowledge to understand and assess your paper.

So, feel free to list reviewers from the wide world! Note that there is typically no obligation for the editor to use the names you suggest; often they will make their own selection, based on their knowledge of the field.