The ansible-galaxy tool can already install things with a requirements file, see --help. It knows about deps already too, even without that. All role deps are automatically downloaded. We're open to enhancements and these should be discussed on ansible-devel
If you want to submit a pull request to somebody else's role it's up to them to merge it in.
If you have your own, it probably doesn't make sense to upload it to Galaxy - that would clutter things up, unless they start to diverge or can't be reconciled.
Internal non-reusable things for personal or private infrastructure probably don't belong on Galaxy.