Neo Queen Serenity
The
information we get from the manga about the future Usagi, or better known as
Neo Queen Serenity are not much. 
She become queen when she was 22 years old after she married Mamoru and got
pregnant. It’s said that she become Queen after she drove away the new
ice age, and with her power and the one of the Silver Imperium Crystal the earth
inhabitants become in a way immortal. She along with her husband and friends
stopped aging when that happened.
She definitely seems more mature then Usagi and the pre-incarnation Princess
Serenity, but there are things that stay the same – even after a millennium,
she still is unable to write kanji well, and that shows in the letter she sends
to Mamoru and Usagi in 20th century.
When she became queen, she lost the power to transform and fight as a senshi. Still it’s said that this is her strongest form (probably only Cosmos form is stronger).
After the attack of the Black Moon, she got protected from the crystal and slept, while waiting for Chibiusa to awaken her own power.
After she woke up, she gave new powers to the Senshi and to Sailor Moon, so she could keep fighting on.
Her dress is almost as the one she wore as princess, but with some modifications,
like the big ribbon at her back and the crown on her head.