Yes. They're all joinable factions in Skyrim. That would be a lame theme though, and it's totally not the same as nonjoinable factions. If the theme were "joinable factions" then all you'd have to do is run through that faction's quest line, wear their gear, and copy the skillset best suited to their work. They're already completely fleshed out, so there's no point building around them.
But for non-joinable factions, the theme is researching, building skill sets, reconciling known faction lore with Skyrim's culture and current events, etc. You're comparing apples and oranges.
Yes. They're all joinable factions in Skyrim. That would be a lame theme though, and it's totally not the same as nonjoinable factions. If the theme were "joinable factions" then all you'd have to do is run through that faction's quest line, wear their gear, and copy the skillset best suited to their work. They're already completely fleshed out, so there's no point building around them.
But for non-joinable factions, the theme is researching, building skill sets, reconciling known faction lore with Skyrim's culture and current events, etc. You're comparing apples and oranges.