It is possible, if Aokiji stays in Blackbeard's sphere, he could face Sanji, but the question arises, would Aokiji be weaker than Shiryu?
I see it, Shiryu should pose a major challenge to Zoro, who after beating Shiryu will reach the level of Admiral or a little more, then and only then could the dynamic that the manga has followed from the beginning take place.
Zoro's opponents are a little stronger than Sanji's opponents.-
so, shiryu could perfectly be admiral level in battle against zoro, and aokiji being a slightly underdog admiral, would be slightly inferior to his zoro opponent, fitting with oda's narrative in zoro and sanji versus.