Midwinter, a variation on Hunter, both are openended.
I think you need to differentiate between openended and nonlinear.
openended = freedom in world exploration, nonlinear = freedom in storyline manipulation.
That said, I'm unable to name a nonlinear Amiga game...
Edit: after a Wiki, I see some use the term "nonlinear" more loosely, merely as "choice". I think perhaps "sandbox" is the more correct term for these type of games, as is shown by this link: