You can choose very much anything you want. The idea of combining plants is to make them compliment each other. You do that by choosing plants that have leaves that look nice when growing side by side. "Look nice" is a very general phrase but this hobby is about what you as an indiviudal like.
In general it is a good idea to combine plants whose leave shape, size, or texture contrast with the other plants growing around them. Or if we look at the leaf color - combine plants that have subtle hues of the same green for example.
Look at some of these pictures and note the contrast or the hue combination. Then make up your mind about what plant you think will look good in your tank. It will have to be a stem plant probably.