All they need to do, is to make AI decision making concerning expeditionary forces better, akka allow majors to tell minors to give exp forces to majors instead of working alone, also, how they "obey" you should depend on their own strength and view on who is their "major" ally, so Dominions should only obey UK, France should not obey UK, nor should USA, and Dominions should not give exp. forces to France or USA, it should be, overall, UK`s decision, on what to do with forces that Dominion give to them.
It mostly works in HOI3 when you set up allied objective on your land and receive the AI troops that it would send towards your direction under your command, only inconvenience is that AI`s forces usually arrive without a proper OOB.
And no, no complete take over should ever be possible, unless it is your puppet. Countries should leave some forces defending their land, or distribute their forces along several theatres, ex. So, for example, Germany get`s to command whatever forces Italy sends to Eastern front, but their own defenses in Italy should not be, unless Italy becomes a puppet of Germany or some sort.
The HOI3 way when I, as France set up objective on my land and receive 10-40 UKs divisions under my command to fight in France, is far, far more convenient and realistic then if I could take control over UK`s forces around the globe.