Well, if we are speaking of canon Dovahkiin, there's no way to know if he did make a pact with any Daedra. The canon Dovahkiin is seen in the Dawnguard trailer as a vampire, but is that the true canon Dovahkiin?
The only way you can truly predict the canon Dovahkiin's fate is by only following the main quest line. After Alduin's death he would most likely become a prominent character in the politics of Skyrim. If the Stormcloaks won the war (this is without the Dovahkiin interfering) then he may become an adviser to High King Ulfric Stormcloak (due to the Stormcloak's dedication to tradition). If the imperials won, I am sure he would be given some sort of office, possibly he would become Jarl of Eastmarch. He would have a family. Upon his death he would go on to be among the honored dead in the eternal fest of the Hall of Valor.
He would be remembered as the Savior of Men, a man of honor and fortitude, a just representative of the law, and a loving husband and father.
What I have said is up for debate.