I'm not the most productive composer there is, but I think the few songs I've managed to complete have actually turned out quite well. I usually write jazz chord melody style songs, or acoustic fingerstyle in open tunings. This allows to take an idea, such as a riff or melody, and really work with it over and over, experimenting with harmonies, bass lines and counter melodies. It usually takes quite some time, but I feel I can let the songs grow naturally, in a way maturing as I work on them.
Lately however, I've been feeling like I want to do something for a band. I'm thinking something along the lines of progg/rock/jazz/fusion instrumental music, quite experimental and far out, yet groovy and accessible. Problem is, when I write, I can never imagine a song without access to the different instruments aside from the guitar in such a way that I get something that feels coherent. I can come up with a riff or melody, but without the context of a bass grove or piano harmony that I want, I feel I can't really work on the part enough for it to really become a piece of music. As of now, I'm just playing around with some phrase for a while, until I get bored with it and move on to something else. When playing a jazz chord melody, I just play the bass line myself, but I can't really lay down a low grove and at the same time play a soaring lead melody high up at the neck.
How do you guys approach writing for several instruments, when all you have to work with is you and your guitar? One reason for my problem might be that I don't have a band. Of course I could try to find one, but that's a bit of the reason I want to write some songs. It's easier to find the right people if you already have somewhat of a vision to show them.