How do I change the tagline input to textarea?

I could be wrong but I don’t think there’s an easy way to modify these fields generally, you’d probably need to use a bit of JavaScript in the admin to manipulate the field when the page loads to change it from an input to textarea.

I’ve not tested this, but something like this in jQuery might work if triggered on page load:

$( '#blogdescription' ).replaceWith( '<textarea id="#blogdescription" name="blogdescription" aria-describedby="tagline-description" class="regular-text">' + $( '#blogdescription' ).val() + '</textarea>' );