I've encountered this situation many times and no one has given me a good answer. Let's say I'm playing in the scale of C. I'm playing a chord progression that goes G F# F C. Each note gets one bar. G is the fifth so I can hit notes in the mixolydian mode. F is the fourth so I can hit notes in the lydian mode. What about the F#? If I wanted to arpeggiate a chord, what chord would it be? Minor? Major? A mix? Other than a C# (fifth), I can't really feel out something good. Help. I need to write a decent bassline.