I don't know about FFNet, but on AO3, you could post each part as a separate story but link them as a series. Then you could flexibly insert each new piece between whichever of the old pieces you felt it fit. You could also rearrange the stories at later points if you need to.
New readers could read through in as complete a format as existed when they begin to read; old readers can subscribe to the series and be notified when a new story is added. Or, alternatively, people could subscribe to you rather than the series and still be aware that this new story was from Series X rather than simply that fandom by the metadata.
I'm not sure how well it would work for old readers who don't have AO3 accounts, but as the invite queue is about 2 days long, that might not be a huge concern.
This is basically what I did once I'd moved my Schirra FFXII timeline to AO3. Previously, on LJ, I gave a handful of the most relevant previously existing stories (before and after the newly completed story) as links at the bottom when posting something new.
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New readers could read through in as complete a format as existed when they begin to read; old readers can subscribe to the series and be notified when a new story is added. Or, alternatively, people could subscribe to you rather than the series and still be aware that this new story was from Series X rather than simply that fandom by the metadata.
I'm not sure how well it would work for old readers who don't have AO3 accounts, but as the invite queue is about 2 days long, that might not be a huge concern.
This is basically what I did once I'd moved my Schirra FFXII timeline to AO3. Previously, on LJ, I gave a handful of the most relevant previously existing stories (before and after the newly completed story) as links at the bottom when posting something new.