considering that most Ballads are love songs with normally a pretty lyric as well as melody it can be hard to write one coming from an instrumental point of view.
I know my first attempts to write something in a quazi-ballad style morphed into other things and I've only written two ballads that I could consider to still be ballads. Subsequently the two are two of my favorite writings.
I guess one thing one could do is write a new melody over something that you are connected to, ie Round Midnight, then take the melody and axe the changes and reharmonize it. Sure that is a round about way to get to a new tune, but it will maybe give you some sort of connection to a piece that you are already in love with.