Why only guitar and bass? You can do some session keys, session piano
Actually then there is also , there is also Horns , Percussion , Strings and Ukulele , but as you have noticed no keys/piano. I think that your wish makes sense. I added the feature request tag to your post so that hopefully someone N.I. will notice this at some point. That still does not guarantee that N.I. will make a Session Piano/keys.
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