There is one material difference to this form of proposal which I have noted elsewhere and which we briefly discussed… making glob expressions opaque (i.e. a special part of the SOURCES property or a $<GLOB:...> genex whose result cannot be inspected by the configure step) would, in theory, allow CMake to resume from the generation step and regenerate only the part of the build system affected by the genex. This would lead to faster regeneration than either the existing explicit file lists or CONFIGURE_DEPENDS-style globbing.
Since the key constraint here is opacity from the rest of the configure step, an interesting alternative could be to add a $<SOURCE_LIST:..> genex which loads sources from a file with a newline-delimited list of sources (or json or whatever, no need to bikeshed). Then when you add a source to the list, it could do a fast regeneration without fully reconfiguring.