Yes, you may think BMW would be a good fit, but you will never get BMW donating a platform for Saab to then engineer for itself.
Trollhatten would be merely a production facility to keep the impression that saab was still "Swedish", and everything else would be done by BMW in Munich...... And all the Saab engineers would defect to Volvo and anyone else :-o
Just look at Rolls Royce and MINI......