Problem: All blocked users are displayed the same block message (MediaWiki:Blockedtext) regardless of whether they are registered or unregistered contributors. This is problematic and does not allow to give blocked users specific instructions based on the block they're facing.
Proposed solution: Split up MediaWiki:Blockedtext into different messages for unregistered contributors and registered contributors.
Who would benefit: Blocked users, giving them the right directions based on the block type (e.g., it makes no sense to advertise for unregistered contributors that they can "email $1" when they can't technically do so).
More comments: Partly done for composite blocks (multiple blocks affecting an IP/IP range) and for partial blocks. See system messages. Done already for global blocks. For Wikimedia wikis, if needed, they could probably be further customised via WikimediaMessages overrides (if there's a need to do so).