The biggest change is that, by finishing off 'R' rather than stopping midseason, 1990s Sailor Moon fanfic will lose one of its customary divergence points. Lots of stories leapt off from the Ayakashi Sisters turning good, because that's where the DiC dub stopped. Rubeus will be much less common of a villain since his fate isn't left hanging. Crystal Tokyo's treatment might also be far different in the fandom, depending on how the dub chooses to translate the vague references to its origin.
In OTL, "Sailor Moon S" was typically treated by fanfic writers as canon backstory, with everything after it and before Crystal Tokyo often ignored. "S" often had that allure that non-exported video games had: I always saw it talked about as the best season of the five. I doubt "Alt S" stories will become common, but post-S stories probably rise in number without "R" as a tale to finish.
I also wonder how Neptune and Uranus will be portrayed in fic. The show has a much higher profile ITTL, and should thus have a mildly bigger fandom than OTL — this still being the mid-90s, with less people online. I'd left the fandom by the time much homophobia apparently started turning up in it regarding their pairing, although that might have been partly fueled by the choice to make them platonic cousins in the English dub creating a dissonance for new fans.
If Sailor Moon: Another Story is brought to America, as has been discussed, that'll be a tremendous influence on fanon, especially for Beryl and Crystal Tokyo.