I cannot emphasise the development of dynamics enough either. Most guys these days forget that the foot is capable of different levels of volume, just as the hands are. As well as the aforementioned, remember that crescendo and diminuendo are good exercise techniques as well as developing dynamic independence of the limbs individually. Rather than learning to play notes, learn how to play those notes. Beater into the head, beater off the head, loud, soft and everything in between and you'll manage to develop expressive single-pedal technique. Heel up and heel down are both critical techniques and you should learn them both and use them appropriately.
Sadly, I think the art of subtlety on the bass drum has largely been lost. Ghost notes on the bass are a great way of adding some spice into a mix. We do it with the snare, why not with the bass? The same goes for the hi hat control foot - different dynamic levels can apply. Learn to incorporate all these into your single-pedal playing and you'll discover you have a considerable expressive edge over other drummers.