A word for: a person you bounce ideas off
Or I should say in this forum: From whom one bounces ideas. Weaker than a co-author or collaborator. Not a confidant (no element of privacy or secrecy). Friend is too broad. Suggestions?
I would say sounding board.
Source: https://www.merriam-webster.com/dictionary/sounding%20board
In an artistic context, muse might fit the bill:
Source: http://www.oed.com/view/Entry/124057 (2.c.)