Create draft and redirect

I wonder if using 'post_status' => 'auto-draft', would work?

That would allow for a post with empty title and content. The problem is it wouldn’t give the URL that is needed for editing e.g. example.com/post_id/edit/ because visiting an auto-draft post results in a 404.

You could put a placeholder in the title, and then remove it again. But I’m not sure if it’s not better to leave it.

I’m not sure if there is any benefit to leaving in the placeholder?

You could add a placeholder then remove it like this:

function custom_mask_empty( $value ) {
    if ( empty( $value ) ) {
        return ' ';
    }
    return $value;
}
add_filter( 'pre_post_title', 'custom_mask_empty' );
add_filter( 'pre_post_content', 'custom_mask_empty' );

function custom_unmask_empty( $data ) {

    if ( ' ' == $data['post_title'] ) {
        $data['post_title'] = '';
    }

    if ( ' ' == $data['post_content'] ) {
        $data['post_content'] = '';
    }

    return $data;
}
add_filter( 'wp_insert_post_data', 'custom_unmask_empty' );

Modified slighty from accepted answer here:
http://wordpress.stackexchange.com/questions/28021/how-to-publish-a-post-with-empty-title-and-empty-content

One issue that arrives as a result of this is when viewing the /edit/ page – your editable title and content regions will be blank. This could be resolved by adding placeholder text (similar to how you would with an empty text form input e.g. placeholder="Title here")

Of course an alternative approach would be to click on ‘create new post’ in the toolbar and have the user redirected to page example.com/new/ where each of the contributable regions are displayed. Obviously these would be empty regions as no post (draft, auto-draft or publish) has yet been created.

Once these regions are completed the user will be free to save (which then creates the post).

The problem with example.com/new/ is that you need to insert a post first anyway, you need it for some things like uploading media.

Humm, didn’t think of that. Auto-draft posts will give you the all-needed post ID. Would you need a scheduled action to remove all unused auto-drafts? I’m thinking for housekeeping?

Great, looks like a solution then! Nice and consistent with how it is done in core too.