Problem: Global block reasons can't use any templates (since the block reason is rendered on the wiki the blocked user is visiting, and all wikis have their own separate template namespace). This means that the user (who might not be the intended target of the block) has to click to some Meta page to see detailed information about the global block that is preventing them from editing. For example, it would be useful to display templates like en:Template:Blocked p2p proxy directly on the blocked error message.
Proposed solution: Implement a way to display pre-defined block reason templates to users by rendering block reasons on meta. Alternatively instead of allowing use of arbitrary meta templates, implement a way to for stewards to configure longer block reasons on a Meta JSON page and selecting one of those on the interface (proposed by Legoktm on the Phabricator ticket).
Who would benefit: Users affected by global blocks (since the instructions they see will be clearer), stewards