Addison

Despite its origin as a patronymic meaning "son of Adam", Addison really caught on for baby girls in the US around the turn of the millennium, following in Madison's footsteps to peak at #11 in 2007 and again in 2010.

But there are plenty of distinguished namesakes for a boy named Addison: from the fictional theater critic Addison DeWitt (All About Eve) to the 18th-century English poet, playwright, politician and essayist Joseph Addison.
It is worth noting too, that Addison remains in usage for boys in the UK and would fit right in with other unisex choices, such as Mackenzie, Hadley, and Ellis.