I'm trying to create 2 different page templates for a website here.
The idea is that admin should be able to select this new template for specific pages at the time of creating it. Also, they should be able to select the old design when ever they want.
What could be the best way to design a new page template with entirely new designs instead of changing any current designs?
Short answer is, yes it's possible.
That is the very purpose of having a template hierarchy. Different templates having different functionality and design based on your needs.
For example, you can create a tag-facebook.php and make it look like Facebook, while having a tag-twitter.php that looks nothing like Facebook, but just like Twitter!
There is one thing you should consider. Most of the page templates have the following structure:
// Header
<?php get_header();?>
// The page's content is here
// Footer
<?php get_footer();?>
If you follow this structure, you can't fully customize your page since get_header() and get_footer() will always include the same header and footer templates in your page.
Unless, you define different headers and footers to use in different designs.
