Air carrier pilot reported the MIA Class B airspace shelf altitudes contribute to conflicts between VFR aircraft with IFR aircraft in the Class B airspace being vectored for initial approach.

Narrative

During vectoring for final approach we received instructions to descend and maintain 3000; expedite descend to 3000 (relative position - downwind to base for RWY 30). Shortly followed by instructions to turn heading 330; intercept LOC 30; descend and maintain to 2000 till established ; cleared for ILS approach RWY 30. As we were at 2000 and intercept the GS (prior to intercept the GS or during descent) we got the aural alert Traffic; Traffic" with a GA aircraft passing underneath of us with approximately 500 ft separation. We did not receive an RA. We did not receive any traffic advisory by the ATC either. After reviewing our profile and Class B MIA terminal area structure we discovered a safety issue. The clearance of the ring over the water on the approach path might look legal on the paper but definitely not safe. Altimeters of the approaching into MIA commercial aircraft might have different settings than passing underneath class B GA traffic. Also most of that GA traffic doesn't even talk to the ATC. This airspace needs to be re designed.Areas of problem into Miami per ATC and pilots:1) over BIRDD (RWY 30) and prior. The inner shelf is 1000/7000. Helicopter or Aircraft under B can be at 500. Prior BIRDD - the shelf is at 1500-7000. The commercial traffic like us are vectored to intercept at 2000 which gives barely 500ft or separation as well. Both of those segments are not safe or practical.2) out 10-20 outer shelf 3000ft-7000ft is not well designed either. Some VFR traffic flies at 2800ft just outside of the 10 mile fixes which MIA approach usually vectors us at 3000. Then the commercial traffic constantly has to be vectored around; get cancelled approach clearances etc. Cause: Redesign class B airspace terminal area around MIA airport"