The promises we do not speak of by Blue Jeans
Immortal cage: an impasse to nowhere land by Blue Jeans
These two connected stories by Blue Jeans are some of the most melancholic Helios/Chibi Usa fanfic I've ever read, but it's written very beautifully and the bittersweet ending had me in want of tissue at the end. ;_;
Forevermore by Nasu Hasami
A modern AU where Lady and Helios are a normal, young married couple. But in the intervening years since their marriage the two have started drifting apart, until one of them could bear it no more. There are small bits of humor sprinkled through out, but for the most part it's a serious contemplation about love and how some times we become complacent with our relationships. I love this fanfic for it's depiction of the ups and downs of a marriage, and it offers an interesting look into what life might've been like for these two had there been no magical powers and they are but two ordinary people working through a low point in their relationship.
Dreaming gods by Singing Muse
A short story about the relationship between Saturn and Helios during the Silver Millennium. It examines the platonic relationship the two had, as two children living outside the world of humans and gods alike.
SailorMoon Crystal Generation by Ame Yumeko
A story I am currently
obsessed with, the author does a fantastic job of not only fleshing out the Sailor Quartet, but also her original characters in this version of the 30th century. It's very hard to describe this story without giving away spoilers, but the insinuations of character's true identities and the mystery surrounding their motives and the truth behind their enemy has me perched on the edge of my seat. There's also a character named Pegasus Kamen, and you know with a name like that things have to be interesting (though I can tell you this Kamen character isn't who you probably think he is).