I don't know if this question was asked before, but:
If, hypothetically, when the Mikdash will be built, somehow it was discovered that all kohanim were genealogically invalidated - chalalim in the lineage, converts, etc.1 What then? Would there be service in the Mikdash? Who would serve?
1 In this hypothetical situation, let's assume that there was a way to be certain of this 100% percent.