A handful of countries, such as Croatia and Spain, already make allowance for such a title; what intrigues me is how many countries do so around the world. For example, El Salvador only has two general ranks; if ES were at war, would it nominate a superior officer to command all the others?
I'm not sure what's being asked. Lots of countries have 5-star equivalents (e.g., United Kingdom), and even use them fairly often (e.g., Egypt). North Korea may be thought to have a 6-star rank. In most countries they've had an honorary air about them. Britain has had many more field-marshals than the United States has had 5-star generals, but most of those field-marshals received the title as a non-functional honour, not as a rank in a warfighting organization.
I don't want to say that the U.S. practice (only practised for a year, I might point out) that the 5-star rank was for a kind of over-general during a total war is unique, but it is nearly so.
The history of the top ranks in the U.S. Army is interesting because of the obvious reluctance to award high rank. As noted, the highest rank in the Army from Independence to the last months of the Civil War was Major-General (tho' General-in-Chief might be thought of as a rank). Even today Major General is the highest permanent rank. And Congress debated fearfully about the 5-star ranks before finally awarding, practically after they might have meant something.
The U.S. Armed Forces today have a kind of covert 5-star rank, sometimes called O-10S for the chiefs of staff and the commanders of the combatant commands.
There was talk a few years back of elevating General David Petraeus to the rank of General of the Army (personally, if we're gonna promote some more people to 5-star rank, I think George S. Patton and Marc Mitscher (both posthumously), Colin Powell, and H. Norman Schwarzkopf should also be considered), and there was a Bill introduced in Congress to posthumously promoted (and about damn time, too!) Admiral Raymond Spruance to Fleet Admiral, but I think Petraeus refused to even consider a fifth star, I don't know if the Spruance Bill passed.
In Sweden, we have only one active four star general, who is the Supreme Commander of the Swedish Armed Forces. Although, during the period 2009-2012, when General Håkan Syrén was chairman of the European Union Military Committee, we had two active Swedish four-star generals.