In peace deals I've noticed on a couple of rare occasions I seem to of had the option of either taking just my opponents capital or demanding other provinces as per normal. Is this a change or is my game going a bit wonky? Only happened once or twice and I didn't pay much attention as I was after specific provinces.
You are allowed to take capitals on some occasions. I know you are allowed to take if it is landlocked (sometimes not?) and not connected to the rest of the nation (whether you have a core or not), and I think there are some instances - like with a two province minor - where you can take a capital. Might need a core.
By far, though, the most common is the first one.