Sometimes having no melody or harmony ideas can lead to some cool stuff. Starting with no pre-conceived notion of what music "should" sound like allows you to more fully explore your creativity.
One of my favorite bands, The Residents, started this way. They essentially had no experience making music and they just kept doing it anyway. They made some music that turned out weird in the best way. I highly recommend checking out some of their stuff. I can give some specific recommendations if that's something you're interested in.