Well it's very difficult to find the exact detail info regarding BIW. Cause I'm working. I was just like you few years back...In general it's just sheetmetal design but that cannot be designed without surface tools (no matter which software you are using) usually mnc / OEM use alias for design(outer - body styling) and CATIA or nx to model the biw for that particular design
As far as it's future. It's never gonna die cause there is always new developments in automobile parts n new technology in electronics used but the structure remains the same and without structure u cannot make it or possibly a CAR in this case
Fixture design is very different from biw
You want learn fixture design or biw?
Fixture for biw is a different subject
BIW is a different subject