Personally, I like the space that happens when the guitarist goes up the dusty end for a solo. To make things interesting you can invert the chord (play the 3rd or 5th instead of just the root), or stick passing notes or accidentals in...
E.G. Chord progression of C, Am, F, G - instead of just playing quavers (8th notes) for each bar of C, A, F, G I'd look at adding a bit more interest and turning the line into a bit more of a feature - rhythmic devices (like the last bar of this example) can also help to make more of a feature of the baseline whilst still underpinning the guitar solo and providing a sense of rhythm too.
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