Well Philipp Melanchthon was the next theologian after Luther died, but keep in mind that the Augsberg Confession and the Book of Concord is the definite declaration of the Lutheran Church (orthodox Lutherans). And Melanchthon himself tried to reconcile Calvinists and Lutherans with regards to the Eucharist and as such wrote the Confessio Augustana Variata or the Altered Augsberg Confession... People then accused him and his followers of being crypto-Calvinists.

The real founders of orthodox Lutheran theology were Johann Gerhard, Abraham Calovius, Martin Chemnitz, Aegidius Hunnius, Leonhard Hutter, Nicolaus Hunnius, Jesper Rasmussen Brochmand, Salomo Glassius, Johann Hülsemann, Johann Conrad Dannhauer, Valerius Herberger, Johannes Andreas Quenstedt, Johann Friedrich König and Johann Wilhelm Baier.

Another thing to remember is that Lutherans look the Book of Concord more than Luther's writings as the authority as to what the church teaches. I've seen Lutherans quote Chemnitz more than Luther when it came to theology.

Peter

If you believe what you like in the gospels, and reject what you don't like, it is not the gospel you believe, but yourself. Augustine of Hippo