Everything's always negotiable, but I ask that you keep this on topic...because adding gray takes away focus on the bigger, more important point being discussed:
Employers own what their employees create.
That's the starting place for any discussion, and if an employee is particularly powerful, brilliant, well-known, whatever it is that the employer values, there are always ways to negotiate.
And THAT'S the important thing to recognize: the starting place for negotiation.
It ISN'T that creators owns what they create.
It IS that employers own what their employees create.
Why is this distinction important?
Because different comics cultures have different knee-jerk reactions to "what's fair," and many don't have "creators owns what they create" as their version of "fair," but "employers own what their employees create" is a constant somewhere at the core of each comics culture.