Hi, Glojo. A good question there. As the external stores are today, they make a massive difference to stealth. It may be possible to design lower observable pylons, etc, but the radar cross section would still look very different to the clean aircraft. So you would effectively lose the LO characteristics and the aircraft would need to fall back on its amazing performance (acceleration and 9g capability). Or not, as it is in the case of the F-35.
The internal weapons bay is crucial to LO.