A project that I would love to undertake if I had more time and linguistic ability would be a study of theological concepts surrounding the root of "letting" (lassen, laisser, etc.). In German the concept of Gelassenheit has been significant for mystical and Pietistic writings since the Middle Ages. Further, it was a concept that was actually examined as a concept quite early, in works like Andreas Carlstadt's Was gesagt ist, Sich gelassen, or Valentin Weigel's Gründlicher Tractat von der wahren Gelassenheit zu Stärckung und Wachsthumb.
The term Verlassenheit is related and also theologically significant (Mein Gott, Mein Gott, warum hast du mich verlassen?), although it hasn't enjoyed quite as much attention for conceptual analysis. In French the word abandonner is usually used instead of laisser for Psalm 22 and Christ's words on the cross, although I did run across the rendering Mon dieu, mon dieu, pour-quoi m'as tu laissé in the psalter begun by Clément Marot and later finished by Theodore Beza.
Luckily for those who are interested in such things, a new work is out on Semantik der Gelassenheit, bringing to print the fruits of a research project that has been going on since 2007. This edited volume primarily covers the medieval mystical tradition, although it does venture a bit into post-Reformation thinkers like Jakob Böhme and Angelus Silesius. It looks like a promising contribution to a more complete understanding of the theological significance of this term over time.