In my opinion its Fruits Basket and Inuyasha as both series have lots of similarities to Sailor Moon, they tend to be the only series I cross over with Sailor Moon as well.
Quite coincidentally, I'm doing a Fruits Basket/Sailor Moon fanfic which pairs Yuki up with Ami.
Right now, I've gotten up to the point where Yuki and Ami attend University together after the Zodiac curse got removed ala the manga and Usagi had given the Senshi free reign as to what they want to do until the time to create Crystal Tokyo arrives.
And I have no
idea how to continue from there.
Additionally, Yuki and Ami had
a past before Sailor Moon Classic happened. Ami voluntarily
allows Hatori to erase her memories just to keep Yuki out of Akito's clutches. Luna's 'Mind Meld' in episode 48 kinda of 'undid' what Hatori did.
By the way, I was wondering if it is possible for Usagi to suffer from the 'Grass is greener on the other side' syndrome; as in she sees the Guardian Senshi happy in their respective relationships and feels that her relationship with Mamoru isn't as wonderful as what the girls have.