Good ideas!

A random idea I had that I never had time to pursue was whether or not this library could move towards a thread-safe solution. As of right now, all promises mutate a global queue and can't be easily used with libraries like pthreads (anecdotally basing this on issues I've seen over the last few years).

I am going to pull out the Utils class changes so we can add this in 1.4.0.

I am going to implement the Utils class for 1.4.0, and deprecate the global functions. That way Guzzle 7.1.0 can use it.

Regarding other changes, like dropping old PHP and adding typing, that can be addressed later down the line, when we are actually ready to start work on 2.0. I want to prioritise fixing the global functions problem on the 1.x series, so we can get that out sooner than 2.0.0.