Although solos are pretty much the melody of a part of a song, a great deal of harmony is important with them and it's there that you really have to consider what you're soloing over. If you're working with mostly one or two chords then things are just not going to be as progressive (so to speak). This doesn't mean that it's not possible to do just that, I've been wrong before and in music there is no right, but if you really think about what it is that you're soloing over and put an equal amount of heart and soul into the rhythm then that'll open up a much larger doorway for ideas. Arpeggios, chords, hell, dual soloing. Certain note combinations will give you different feelings and sounds and whether you use notes that are in the chord underneath, out of the chord under or even flat out of key is all just an option you can use for a sound you want to perceive. A solo is like writing a fine piece of literature. If you only write one draft then the chances are it will sound ho hum or uninspired, but if you write it down and you read over it and rewrite it then it will gradually improve and become much more in depth and will have a greater impact. Scales, in my opinion, are good to know and help with guide lines, but to me they're just that. Guidelines, but when it comes to making something from the heart and perfect to me, I'm not about to limit myself to any selection of notes, chords, keys or anything. I think that shutting ideas out is rash and can greatly limit the progress a song can make. Keep in mind, I'm expressing a lot of opinions and those can only go so far beyond me, so don't go running with them. Best thing you can do is just find a piece for a rhythm and record a solo over it, listen to it, record another over it and compare. And continue this process as you see fit. And really pay attention to what you're doing too, don't just hit random notes. Think about what would sound best, keep up with your playing and keep an eye on any movement that felt good to you. And with that, keep practicing.