I remember what film critic Ebert once said when reviewing the original Back to the Future. How do we know that the original timeline really was the original timeline, or if there even was an original timeline? "That's not a job for Superman, that's a job for God."
In my head cannon, there was only ever one timeline. That it might be threatened is one thing but it was always there. Otherwise, its the proverbial can of worms. I'll be going with the anime because that's the one I know...
After the Doom Tree, SM and the gang get a long well deserved rest because the Black Moon/Negamoon never shows up. Tomoe and crew would still be there eventually but without Rini / Chibi Usa's pure heart Mistress Nine would never arise... or would she. She could get someone else's heart theoretically but...
Sailor Pluto came to the 20th century to make sure the timeline ran smoothly but there would be no need of that here. There would have been no BMC to shake up the timeline. No Pluto means no friend for Rini and Hotaru. Uranus and Neptune were willing to kill Hotaru to prevent the rise of Saturn--Pluto, on the other hand, while an outer, always seemed reluctant.
Might Neptune and Uranus actually kill Hotaru? Might that be the reason that we never saw any of them in Rini's flashbacks? In the original timeline, the two outers killed a child and for that crime, they were forever banished from NQS presence?
The next seasons, I can't say but the implications of the above is a doozy.