We'll create fresh WordPress site with DX Template Manager installed. You have 20 minutes to test the plugin after that site we'll be deleted.
Create page templates like the ones in your theme folder but through a “DX Templates” menu in your Admin dashboard. Paste HTML, JS and PHP code which you could assign to your posts, pages or custom post types via a meta box dropdown. Create page templates and apply them to be evaluated.
Note: eval() function is used. However, it is available only for admin users to submit code and normally admin users could do a lot harm or upload external harmful plugins as well.
A complete demo is available here: