…The only criteria that is required to be a Sunni Caliph is to be elected. That’s all.
"Main article: Succession to Muhammad
In his book The Early Islamic Conquests (1981), Fred Donner argues that the standard Arabian practice at the time was for the prominent men of a kinship group, or tribe, to gather after a leader’s death and elect a leader from amongst themselves. There was no specified procedure for this shura or consultation. Candidates were usually, but not necessarily, from the same lineage as the deceased leader. Capable men who would lead well were preferred over an ineffectual heir."
en.m.wikipedia.org/wiki/Caliph
…the fact that the Sunni have not chosen a Caliph immediately after the death of the last one means they have broken tradition. They are unorthodox.