One great 'Yugioh Female Hikari' fanfiction is 'Hikari's Journey'. It focuses on Hikari's growth as a duelist in the Yugioh world. The story has really well - developed characters and interesting duels.
Yes, there are likely some fanfictions out there about Hikari and Kei getting married in the Special A universe. You can try searching on popular fanfiction websites like Fanfiction.net or Archive of Our Own. Just type in relevant keywords such as 'Special A Hikari Kei married' and you might find some interesting reads.