We've done that to some extent via the legal enablement of nurse practitioner and physician assistant led care. Of course, largely speaking all they do is supervise the recording of patient metrics and prescribe drugs in label-consistent ways, but that often works out reasonably well for the patient. When the patient needs specialty care then the NP or PA simply punts them into the winds of referrals and insurance justifications.
I'm not sure there's any realistic way to enhance the availability of specialists. You can't 'stub' your way through providing the care of a skilled gastroenterologist by substitution with a NP, though PAs in specialty care are becoming common.