Replacement for wp_title()?

title-tag.php

<?php

// In a theme:
add_action( 'after_setup_theme', function() {
	add_theme_support( 'title-tag', array( 'option' => true ) );
} );

// In core:
add_action( 'wp_head', '_wp_render_title_tag' );
function _wp_render_title_tag() {
	if ( ! current_theme_supports( 'title-tag' ) )
		return;

	if ( null !== $pre = apply_filters( 'pre_wp_title_tag', null ) ) {
		echo '<title>' . $pre . "<title>\n";
		return;
	}

	$options = get_theme_support( 'title-tag' );
	$options = $options[0];

	// do something special to generate $title

	$title = apply_filters( 'wp_title_tag', $title, $options );
	echo '<title>' . $title . "</title>\n";
}