There's one thing I'd really like to see there: the ability to lock out the repository and perform a really aggressive repack. I'm talking `-AdF --window=500` or somesuch. On $dayjob's repository, the base checkout is several gigs. Aggressively repacking it reduces its size by 60%.
There's also a git-level thing which would greatly benefit large repositories: for packs to be easier to craft and more reliably kept, so it's easier to e.g. segregate assets into packs and not bother compressing that, or segregate l10n files separately from the code and run a more expensive compression scheme on that.