Hey,
When I was looking in to progressing from the simpler stuff it was really listening and playing to red hot chilli peppers that pushed me. I recommend finding a bass player who really calls out to you and copping all his licks. I would also say, make your ears really big! By which I mean listen to as much music as possible. Programmes like spotify are making this so easy now, take advantage. Stevie Wonder, Miles Davis, James Brown are pretty cool artists too look into when looking to open your ears.
Also check out this series of podcasts by Janek Gwizdala: [url="http://gwizmon.podbean.com/"]http://gwizmon.podbean.com/[/url]
It may give you some insight into the process of improvisation, its pretty advanced stuff however. Ultimately listening to a diverse range of music is going to push your abilities, and keep having fun.
