Nan-
Once you've equipped yourself with some or all of this great stuff your getting try this.
Identify in a particular song that you want to improve the music leading up to the point you want to place a fill. If you've got a recording great but just playing it over in your mind will work to. Imagine to get into the feel of the song, and let your mind come up with a fill or two, etc. I'm all the time hearing fills that would sound and fit well within songs, I don't always work them out, but that could be another starting point. Imagine the sound of the fill you wish to achieve then try working it out. Eventually you'll do this as you play as one mentioned before- you'll be playing and depending on the mood of the song, the band, the audience, all that "FEEL" can lend itself to dictating some great spur of the moment fills. Maybe fast and furious, maybe stay in the pocket, maybe some real expressive, full but simple fills as in some power ballads POWER UNDER SUBMISSION- you could do more but just like a well trained Arabian stallion you keep yourself in control with the spirit or feel of the song.
to jaedrum & MVanDoren1, THANK YOU for sharing your views as well. Blessings.
Nan