The stamp is currently 'In Design Development' as are many of the others destined to arrive in 2015. You can see the full list of approved subject matters in the images below. There's also a list of the stamps that will be coming out this year.
The WP notes that the approved subjects typically include traditional, historical, cultural, and literary figures; however, the financially strapped service is looking to increase revenue and attract young collectors with more popular subjects.
Read More [via Jamie]