The Roman Empire is one of the most studied subjects in human history and one of the most popular. School curricula, university programmes, documentaries, podcasts, games, and a general readership that has never diminished in five hundred years of printing.
The definite article matters here. It makes the name read as the subject itself rather than as a description, which is the right register for an authoritative resource. It sounds like the reference rather than a site about the reference.
On the .org it fits an educational resource, a museum or archaeological project, a learned society, a digital archive, or an academic publisher. Classical studies live almost entirely on this extension.
The content opportunity is real. Roman history is taught everywhere and served online by a mix of ageing university pages and thin content. Something properly built would find its audience quickly.
Three words, exact subject, no hyphen, and no risk of the topic going out of fashion.
The name is listed. Enquiries welcome.