Here are some great fills for the open A, D and E chords. I’m mainly basing these off of the “sus” chords and the A major open scale. These should give you ideas to experiment on your own. A lot of the “riffing” around open chords come from the scale and sus chords! I have a tab available below. Try playing along to these with one of my backing tracks!
0:34 – Open Major Scales
0:54 – Sus Chords | BO 8/10
0:54 – Playing Open Chords | BO 4/10
Open Scales, Chords & Acoustic Fills | 2 – A-D-E Tab