World leaders should not be afforded any form of preferential treatment. Everyone is equal under the law; all users equal under a platform. That's it. The moment you treat a subset of the population differently from another (such as giving them a blue checkmark), you begin to break from this ideal. Of course, any platform is free to do as they please. But congrats, in doing so, you have devolved into a world of kings…
It's a pretty far cry from having any sort of additional power.
Furthermore, acknowledging status where it exists is not the same as creating a distinct group of people with special status. What about "Dr." for people with doctorate degrees? Doctors don't have any more rights than anyone else, and having the title exist certainly doesn't create some part of society that is inaccessible to others.
Holding political leaders to a different standard on social media doesn't mean they are royalty, it's just an acknowledgement that they do hold power, elected, inherited, or otherwise. Furthermore, if Twitter's history is a sign of what any new policy would look like, world leaders would be held to a more restrictive standard than others, on Twitter.