First of all, I hesitate to state this here, but the current Logos incarnation of the Kolb/Wengert edition of the Book of Concord is an embarrassment. The tagging seems inconsistent and in addition the references under the article level are not even displayed. This combination makes it quite difficult to use.
It would be great to add the Sources and Contexts book edited by Kolb and Nestingen, as well as the new History and Theology of the Book of Concord by Kolb, Arand, and Nestingen.
While many classic works from the 16th century are available, it would be nice to add a more complete collection of Sermons of Martin Luther, as well as his Letters of Spiritual Counsel. In addition the 1521 Loci of Melanchthon would be nice, even if a later edition is available.
For Luther's biography, the 3 volumes of Brecht would be good.
My body, the Lutheran Church Missouri Synod, has offered decent collections about Lutheran Orthodoxy. But there are few resources for Lutheran Pietism, in its various forms - in spite of its continuing influence. Ardnt's True Christianity and Spener's Pia Desideria leap to my mind. But much of the continuing influence of Pietism is in hymns... At the very least, there should be a collection of Paul Gerhard. Maybe the Lyra Germanica of Winkworth.
Speaking of Hymns, a collection of Grundtvig would be good.
Missouri has been pretty good at releasing the most important 19th century works form her history. In addition, many biblical works have been added because of general interest (eg. Hengstenberg, K&D) But some important books like von Harless's Christian Ethics and Loehe's Three Books about the Church are missing.
From the German Church Struggle, you now have Bonhoeffer (yippee!!) but I wish Schlink's Theology of the Lutheran Confessions was available, as well as von Rad's commentaries on Genesis and Deuteronomy.
Some 20th century introductions include Hermann Sasse's Here We Stand, Gerhard Forde's Where God Meets Man, Gritsch and Jensen's Lutheranism, Braaten's Principles of Lutheran Theology,
The American Lutheran/Roman Catholic dialogue reports and ESPECIALLY the study documents are wonderful and would be welcome in Logos.