Addison has been a wildly popular surname name, mostly for girls, for a few decades now, and Addisyn is one of its many spelling variations.

Addisyn

Allisyn

Though it relates to the Irish original, Ashlyn and its next most popular form, Ashlynn, owe more of their popularity as baby names in the US to the megastar Ashley, though all three have been dropping on the charts lately.

Ashlyn

As Ashley becomes one of today's "mom names" thanks to its popularity amongst millennial-borns, Ashton is a fresh alternative. This particular spelling is nearly completely equally split between girls and boys in 2022.

Ashtyn

Bryn is an up-and-coming gentle, yet substantial, Welsh name that would also be effective in the middle spot. In Wales it's a traditional boys' name, but it's far more popular for girls now in the US.
 Bryn entered the U.S. girls' popularity list in 1999 but is outranked by the Brynn spelling. Reality star Bethenny Frankel named her daughter Bryn in 2010.
 Trivia tidbit: Bryn Mawr college was the first institute of higher learning to offer graduate degrees to women.

Bryn

Caitlyn is following all variations on the Irish classic Caitlin down the US baby name charts, following decades of popular usage. The original Anglicized form Kathleen is now starting to feel fresher than these more modern forms. 

 Transgender athlete and former Kardashian spouse Caitlyn Jenner chose this spelling for her post-transition name in 2015.

Caitlyn

There are 20 times as many baby boys named Camden as baby girls, but only twice as many boys given the Camdyn spelling as girls, making this version a much more gender-neutral choice.

Camdyn

This Cameron variation was popularized by actress Camryn Manheim and the character inTwitches of the same name. It also makes it more obviously feminine, which might appeal to some parents, considering the classic Cameron is a Top 100 choice for boys in the US. 
 
Deriving from a Scottish surname made up of the Gaelic elements cam and sròn, Cameron means “crooked nose”. This likely originated as a nickname, but if you were hoping for a different meaning, there is also the theory that the second part of the name may have come from the Gaelic abhainn meaning "river" or the Welsh bryn meaning "hill"
 
Cameron, Kamryn, and Camryn are all in the US Top 1000 for girls, with Camryn given to around 600 girls each year.

Camryn

Modernized spelling not enough to revive Karen. 

Caryn

Cathryn

As frozen in the pre-Beatles era as short white gloves. 

Cheryl

Devyn

Eryn

After decades of disuse, soft and feminine Evelyn has returned to the baby name stage in a huge way. It reached the Top 10 for the first time in 2017. Evelyn has now surpassed its former heights, joining a legion of contemporary little Evas, Avas, Eves, Evies, and Evelines. 
 
Evelyn derives from the French feminine given name Aveline, which is from an obscure Germanic root which may mean "desired, wished for" or "water, island". The name Aveline was brought over to England by the Normans, but it first became popular as a masculine name – a transferred use of the surname Evelyn, which comes from the same source. Variations include Evaline, Evalyn, Evelin, and Eveline.
 
Formerly a predominantly masculine name, its most famous bearer is writer Evelyn (EEV-lin) Waugh. (Trivia note: Evelyn Waugh was married to a woman named Evelyn, and they were known as 'He-velyn" and "She-velyn'.) Its quiet persona was reversed with the character Evelyn Salt, played by Angelina Jolie. Bruce Willis chose it for his fifth daughter and Carey Mulligan used it for hers.

Evelyn

Evyn

Fynn

For girls, the most popular form of a very popular name.  See also Jaidyn, Jaiden, and Jaden.

Jadyn

Kaylin and all of its forms have made many appearances in the Top 1000 in recent years. Kailyn is also the Turkish goddess of crossroads, kings and queens.

Kailyn

Although the Kathryn spelling feels like a modern streamlining of this ancient royal and saints' name, it is in fact found back through history. At least one of Henry the Eighth's unfortunately wives sometimes spelled her name as Kathryn.
 
While Katherine is the number one spelling of the name these days and Catherine number 2, the Kathryn form is well-used if not exactly modern-feeling. Two recent Kathryns of note: Academy Award-winning film director Kathryn Bigelow and bestselling novelist Kathryn Stockett.

Kathryn

Kellyn

Given a new shot of style by singer Lauryn Hill, this spelling of the name has fallen quite a bit in popularity since its peak of 167 in 1996.

Lauryn

Londyn is now a more popular name than the original London for girls.  Some parents feel that varying the spelling nudges the name away from over-identification with the British capitol. Substituting a y for another vowel is also seen by some as more feminine. For boys, London is five times as popular as Londyn -- though both variations are far more popular for girls.

Londyn

Landyn

Lennyx

Macyn

Better known than it would otherwise be thanks to Meryl (born Mary Louise) Streep. 

Meryl

Morgyn

Maddisyn

Rowyn

Tristyn

Taylyn

courtneylynn4's list "Modern Girl Names with a Y twist" of 31 great name ideas: Addisyn - Taylyn!