quoted
Perhaps the default setting of submodule.<name>.ignore should be 'untracked'?
I still vote for none. I think the default should be to not have untracked files in
your projects (like you should not have warnings when compiling your project).
If that is not wanted, just use the configuration options git provides to change it.
I forgot to mention: I think untracked files should mark a submodule as modified
because otherwise it is too easy to forget adding new files inside submodules,
as they won't show up when the 'ignore' setting is 'untracked' .