I've got myself a go-to ensemble that I use which is a six-piece rock band (2 guitars, 2 keyboards, bass, and drums), string section, and electronics - it's the ensemble I use for Verdris Moth, a studio project of mine. I play with the instrumentation a bit - sometimes I add an extra keyboard or an instrument I don't use all the time like a koto or harp or whatever, and sometimes I strip it right down and just use piano and strings for example, or keyboards and guitars only. Like for instance I didn't feel like drums would have worked on my music for this jam so I omitted those, but if it was for an album for its own sake, I might have stuck some big drum fills in there or something. Depends entirely on what the project is!
I've been mucking about with all of these different instruments and styles and techniques for years, since well back into my teen years, deciding what musical things I really like and would regret not including. For me personally I'm into things like weird chord sequences, memorable melodies, recurring and developing motifs, unexpected structures, and style-wise I'm most influenced by prog rock, film music, jazz, funk, ambient electronica, and a few other things. But it's taken ages to get to a stage where I feel comfortable just coming up with a melody and knowing what sort of things I might do with it.
So for this jam a melody came up, I tried it on piano first, then tried a built-up version with my usual ensemble, and I had something basically there albeit in need of a good polish and fleshing out.
Going on instinct and not thinking too hard about what I'm writing tends to work best for me, then later on I can screw around with my ideas, or shelf them and wait for a more appropriate project to use them on, or indeed just get rid of them altogether if they're not working at all!