There's been many hypotheticals involving conversion of outside peoples beyond the core Jewish-Hebrew ethnic group(s) to Judaism. Most Jews today have some degree of descent from the historic population of Palestine, particularly on the male side of the gene pool, and the lack of converts and how converts out of judaism tended to leave the communities entirely tended to compound the ethnic identity.
Had Judaism spread further and stuck, whether it be via a successful Himyarite Kingdom, Khazars promoting Judaism beyond their nobility, the berber communities who had converted remaining Jewish rather than converting to Christianity and Islam down the line, etc - what would have been the impact on Jewish identity as far as ethnicity goes?