Hey OP I don't think I've seen this mentioned yet, but one important thing to consider is the collection of PSRs (PHP Standard Recommendations), several of which are around HTTP clients.

This is an important component to decoupling packages from specific underlying implementations like Guzzle.

You can even use the auto-discovery package to have whichever conforming HTTP client is installed (like Guzzle for example) be utilized, or allow via dependency injection an existing HTTP client to be passed in.

This would be a great enhancement since the PHP community has done a great job with these shared standards, it is a common expectation across packages to utilize them.