Idea
I would love to have a feature in Laravel that enables us as developers to auto tag all the classes that implement an interface or extend a certain class.

The behaviour of this would be comparable with autoconfiguring tags from Symfony.

Inner workings
For a project a made a PoC (Proof of Concept) for this where I used the classmapper from Composer to scan all the classes within a namespace (the namespace could be provided through a configuration). If the class extends / implements the expected class/interface, its service would receive the provided tag.

In the code the user could do something like the following in their service provider / app:

$this->app->tagInstanceof(BaseServiceTypeInterface::class, 'my-tag');

$this->app
    ->when(MyDynamicHandlerService::class)
    ->needs(BaseServiceTypeInterface::class)
    ->giveTagged('my-tag');

// ...

class MyDynamicHandlerService
{
    public function __construct(BaseServiceTypeInterface ...$handlers)
    {
        $this->handlers = $handlers;
    }

    public function handle(string $type, array $config)
    {
        foreach ($this->handlers as $handler) {
            if (! $handler->supports($type)) {
                continue;
            }

            return $handler->handle($config);
        }

        throw new \LogicException('Missing handler for type '.$type);
    }
}

The performance penalty of doing this on runtime seemed to be marginal (less then 0.1 sec on a large application (+1700 classes), but I would recommend to make in the PoC I introduced a caching mechanism which prevented rerunning the same code when nothing changed.

Why would I use this
The use case for me was that I was implementing a service which retrieved a couple of handlers following the strategy pattern. This functionality enabled me to quickly add and remove handlers without having to manually declare them in a service provider or config.