A common "out" when the studs aren't in the right place for the thing you want to hang is a "backer board" that spans a couple of studs or that can have multiple "panel fasteners" - with the intended object mounted to (or hung on) that. Making a "decorative" board is simpler, and a lot more fun than breaking out, cleaning up, and replacing studs and wallboard. When you get done, call it an "escutcheon" to make it sound like you knew what you were doing.

Adding a stud in some of my walls would be a bit of a bother, even if not really difficult, due to the change in "stud standards" from 1.87 x 3.87 inch to 1.75 x 3.75 inch for the "2x4" since some of the walls were put up.

In my market, "stud length" for precuts now - which usually are cheaper than standard 2x4s - is also "1.75*3+1.5 less than 8'" ( = 92.62") rather than the old "1.87*3+1.5 less than" ( = 88.88), although it's a good idea to check what you really need each time before checking what you're actually getting, since not all "standard dimension" stuff is accurately "standard." Of course you can cut one that's too long so that it's shorter; but it's difficult to cut a short one off to make it longer.

John