Here is some early concept art for Kimi no Na wa.
The working title is given in the top-left corner of this image. What was the title?
(This question has been gleefully lifted from reddit.)
The title is 「夢と知りせば」 Yume to shiriseba, which one might translate as "Had I known it was a dream".
To appreciate the significance of this title, one should know that this is part of a tanka by Ono no Komachi, a Heian-era poet. In Japanese, the text is:
There are many scholarly translations of this poem; here is Donald Keene's version:
Thinking about him
I slept, only to have him
Appear before me―
Had I known it was a dream
I should never have wakened.
Shinkai mentions this poem as an inspiration in this interview:
As a side note, the text beneath the working title reads ―男女とりかえばや物語 - roughly, "boy and girl Torikaebaya Monogatari". The Torikaebaya Monogatari ("The Changelings") is another Heian-era work, about a boy and girl who each behave as members of the opposite gender. That's basically what Kimi no Na wa is, after all, except with full-on body-swapping.