C-interop, specifically what are some weird, non-obvious, and powerful things you can do, as well as improvements in F2018;
Working with legacy FORTRAN, especially FORTRAN IV and earlier, and vendor extensions. Especially the esoteric, inscrutable stuff;
Fortran history, specifically what were computers like in the 50’s and 60’s, what was it like to be a Fortran programmer back then, early programming languages that were influenced by Fortran, and similar;