Regarding user-input modules, I’d err on the side of simpler (minimal) and allow user modules in the same source as the program unit.
Otherwise, enabling stdlib (via, say, a check-box/radio) or other fpm packages from fpm-registry (via, say, a drop-down menu) would IMO be a killer feature for quickly trying out a package or an stdlib feature. With current fpm capability, it doesn’t seem to me like it would add much complexity on the backend. This could be added to the list of features from which to prioritize. It’s currently not clear to me how high of a priority this should take over some other features.