When I work with others I let them know up front that it is their song, not mine, and they have the final say. I have no problem with them saying this isn't what they want. When collaborating one can not have an ego problem. I done stuff that others have loved and I have done stuff that others have hated. If you have thin skin then don't collaborate.
I've worked with Mario and other guitar players online (here and other forums/sources).
I agree with him; look for a volunteer first. This will hone your skills at collaboration and (maybe) negotiation. Plus you may find a piece of gold along the way.
Some experiments/experiences may not end as expected, but you do get to learn from every one.
/I've personally used things that Mario contributed and discarded others, and it has never been a problem. Not all musicians have an ego. <grin>