There are a lot of different kinds of Series in MusicBrainz, ranging from,

1. "official" series that have a central authority that defines what is (and is not) part of the Series. For example,
   • FINAL FANTASY XIV Original Soundtrack (mirrors a section of an official website)
   • The dozens of series listed at the bottom of https://columbia.jp/idolmaster/ 
2. fan-curated series, where what is a part of the series is highly subjective.  These "series" range from having official numbering to no reasonable numbering/serial property, as well as a range of strength of a common theme. For example,
   • Doctor Who soundtracks (official releases probably, but the numbering property looks like it's fan-curated, so I would consider it to be "fan-curated" rather than "official")
   • Nintendo (very weak common theme, no reasonable number property. I'm not sure I'd even consider this to be a "franchise" rather than a "series")
   • There's also series like Death by heart failure - should this just be a tag instead?

I think there should be a property/attribute on each Series that indicates whether it is,

1. "Official" (i.e., defined by a central authority)
2. unofficial/fan-curated (subjective)