While not directly part of creating a new post initially, I'd love to see the experience around nonces expiring improved. When you are writing a long post and leave the add post page open too long and try to save things, seeing a bunch of -1s and "are you sure you want to do this?" messages is frustrating. There is a ticket for auto-refreshing nonces for the customizer:

https://core.trac.wordpress.org/ticket/31897

but anyone know of any other work in that part of the post editor?