In which particular ways musically and technically, what kind of stuff do you listen to and like and where do you want to go with it?
Try this Bach Cello Suite from Cliff Engel's site, it is a cool musical piece for practice, involves quite a bit of string crossing and spanning, also people dig it because they recognise it and like to listen. I play it in standard tuning, not cello, the tab included is for cello tuning, not too difficult to get around reading-wise but harder in standard tuning to play, trickier fingerings.
http://www.cliffengel.com/bass_lesso...olo_bass/bach/