When it comes to chordal variations the accompanist is not as limited as has been suggested,firstly he can try different inversions of the same chord,there are loads of different e7,if the key is a major,and at least four A major chords., at leastThree different D Major,at least three different g major.
Plus if in open tunings many different D modal ,g modal dyads, A7. In standard tuning this applies to a lesser extent,But there are still a number of options,substitution of dominant 7,with dominant 11,or D min[with doubled third] for g7,sometimes will work.
Of course this is easier if you are the only accompanist.Dick Miles