Just an idea, to have routes assigned to variables to make routes more dynamic.
I come up with this while was changing bunch of same URLs in the twig template.

Let's say we have this route with 3rd argument as a variable:
$router->add('posts/index', ['controller' => 'Posts', 'action' => 'index'],'post_index');

We can then pass it the same way as route_params to the controller and in addition create a global twig variable (called routes) so it can be accessed from the view.

and instead of having this in twig:
<a href="{{ URL }}posts/index">Test</a>
We could have:
<a href="{{ URL }}{{ routes.post_index }}">Test</a>
Which after render would give
<a href="https://website.com/posts/index">Test</a>

With this solution we'd need only change route in one place, instead of going through bunch of files.

I haven't thought of implementation of it yet though, so if I come up with something - will let you know