In no particular order:
1.
Chibi-Usa and the adventures of her Quartet ft Saturn in Crystal Tokyo obvs
2. A
Sailor V spin-off of course.
3. Kind of an unusual answer but I'd personally love to see
a spin-off of Kaitou Ace separate from Sailor V's. He always struck me as one of the more interesting villains Naoko wrote and there's so much mystery surrounding him. For example how did the reincarnation of this low-ranking no-name civilian happen to get his memories of the past back before anyone else? What was his civilian identity before joining the Dark Kingdom and founding the Dark Agency? Did he even
have one? How'd Beryl manage to recruit him anyways?
4.
Manga!Galaxia spin off. I'd love to see the details of her origin story and her rise to power, maybe this spin-off could also include more information on the twin Senshi Lethe & Mneosyne and their planets.
5.
Outers spinoff specifically of HaruMichi (Though isn't that just basically Utena?
) but the day to day lives of an Outers family spin-off could work too.
6.
Naru spin-off. As the ultimate victim in the SM universe (at least 90's anime wise) let's get her perspective for once
7.
90's anime Black Moon Clan spin-off exploring their lives on Nemesis and shedding more light on just what exactly their ancestors needed to even be cleansed
from.
8.
90's anime Dark Kingdom spin-off. Please give us more information on the political inter-dynamics that seem to be at play in Beryl's kingdom, for example what's the history between Jadeite & Tetis? Why does Zoicite harbor such a grudge against Nephrite? How did Zoisite & Kunzite manage to fall in love? There's so much material here that's worth exploring. Could also do a spin-off based on the manga version of Queen Beryl and following her life from a Terran court sorceress to her reborn life as a researcher culminating in getting her memories back and reviving/founding the Dark Kingdom. In this spin-off we might also be able to see the reincarnations of the Shittenou and the kind of lives they were leading before Beryl got to them again.
9. I'll be honest, a
Tuxedo Mask spin-off is one that never struck my mind until this thread but in a sense I can kinda see it? Maybe if it's in the manga or PGSM verse we could get more of Tuxedo Mask's jewelry heists pre Sailor Moon's awakening and see him duke it out often with Sailor V (maybe in a friendly rivalry?) Even a spin-off exploring Mamoru's time growing up in an orphanage would be interesting, or maybe a SilMil based one from the perspective of the Golden Kingdom following the adventures of Mamoru & the Shittenou ft Helios. (Could even do something in modern day with Mamoru & his boys by trying to revive the Shittenou)
10. I know they were just filler characters, but an
Ail & En spin-off of them wandering space trying to restore the Makai Tree the
right way this time sounds rad to me. Could even have it take place in the past prior to them coming to Earth and follow their wacky adventures attempting to integrate into various planets in their attempts to steal energy.