I think a lot of people in this thread understand coming "back from extinction" as more the hypothetical "what would happen if a species just poofed back into existence" rather than the more plausible "what would happen if scientists used genome engineering to recreate a species".

Realistically, most cases of de-extinction would require editing the genome if a living relative in ES culture and then cloning the engineered genome using standard somatic cell nuclear transfer into an oocyte.

If you have the capability to make the millions of genome edits necessary to reconstruct and entire genome, you have the capability of engineering intraspecific genetic diversity as well. The latter task is the easy part. Reconstructing a genome in the first place is an immense task.

Any species that's a de-extinction candidate by definition has museum/historical/ancient tissue samples with sufficiently high quality DNA available. This can be a library for intraspecific diversity. Small amounts of cross-species diversity may also be viable.