The creation of new counties (and county equivalents) is pretty rare. Are there currently any serious possibilties of new counties being created? New county movements are fairly common, but they rarely materialize.
Some recent examples:
I believe Broomfield, CO (city-county) is the newest county in the United States, having been created in 2001. La Paz, AZ is also fairly recent (1983).
Even changing county borders seems fairly uncommon. Colorado (again!) did this fairly recently, with Denver (another city-county) annexing land from Adams County for the airport. The independent city thing in Virginia has probably had some changes---I know the cities occasionally re-join counties.
Alaska has probably done some new boroughs recently and will continue to do so in the near future. That's not as interesting though...
It would make sense to split some of the California counties like those that run between Sacramento and Nevada, and also Riverside and San Bernardino.
In Texas, Potter and Randall should merge.
In Colorado, a county commissioner from the southern part of the county has to drive through 6 county seats to get to his own county seat (it is only 4 in summer).
Within cities, counties don't necessarily have that much authority. Broomfield was somewhat special because it was in 4 counties, and fairly removed from any of the county seats (Boulder, Greeley, Brighton, and Golden). Aurora might consider becoming a city and county.
There are possibly some rural areas like most of Park, eastern Elbert, eastern Adams and Arapahoe, except they really don't have the population to support a county government.