Proposal to improve replies

Relevant issues (see MSC for details):
* https://github.com/matrix-org/matrix-doc/issues/1661
* https://github.com/matrix-org/matrix-doc/issues/1541
* https://github.com/matrix-org/matrix-doc/issues/1654

proposals/0000-reply-improvements.md

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+# MSC0000: Improving replies
+
+Currently replies in the spec require clients to parse HTML and/or plain text in order to accurately
+represent the event. This fallback also introduces a number of limitations preventing clients from
+typically being able to reply to non-text events, such as state events or images.
+
+This proposal alters the requirements for replies such that they can be expanded to cover any event
+type, and be easier for clients to render.
+
+For context, the current spec is extensively documented here: https://matrix.org/docs/spec/client_server/r0.6.1#rich-replies
+
+This fixes https://github.com/matrix-org/matrix-doc/issues/1541 and would technically address
+https://github.com/matrix-org/matrix-doc/issues/1661 as well due to not defining a strict format.
+
+## Proposal
+
+Firstly, building off the concepts of [MSC1849](https://github.com/matrix-org/matrix-doc/pull/1849)
+the relationship between events in the context of replies changes to be:
+
+```json
+{
+  "m.relationship": {
+    "rel_type": "m.reference",
+    "event_id": "$example"
+  }
+}
+```
+
+The fallback for replies is simply replaced by a best effort clause: clients SHOULD do their
+best to provide context of what happened in the `body` and `formatted_body` for clients which
+do not support replies. This could take the form of a simple blockquote, similar to quoting a
+message.
+
+Instead of having to parse the fallback to get the reply, clients MUST send a `reply_body`
+(and optionally a `reply_formatted_body`) in the event content to represent what the user is
+replying with.
+
+In an example, if someone were to send a message like "Version 1.1.0 is out!" and another were
+to reply to it, the reply's content would look like this:
+
+```json
+{
+  "body": "> Version 1.1.0 is out!\n\nAwesome, thanks for the hard work!",
+  "format": "org.matrix.custom.html",
+  "formatted_body":"<blockquote>Version 1.1.0 is out!</blockquote><p>Awesome, thanks for the hard work!</p>",
+  "reply_body": "Awesome, thanks for the hard work!",
+  "reply_fomatted_body": "<p>Awesome, thanks for the hard work!</p>",
+  "m.relationship": {
+    "rel_type": "m.reference",
+    "event_id": "$example"
+  }
+}
+```
+
+The `format` field describes the `reply_formatted_body` format.
+
+## Potential issues
+
+Events will be larger as a result, though typical messaging scenarios hardly get anywhere near the
+event size limit.
+
+There is also an opportunity for the fallback and the `reply_*` fields to desync, though this is
+largely believed to be a client bug rather than a fault of the spec.
+
+Some events, like images, cannot be represented easily as text. Clients could show something like
+"> regarding Bob's image: release-timeline.png" instead.
+
+## Alternatives
+
+We could ditch the idea of a fallback entirely, leaving clients which don't support the feature
+in the dust. This is a viable option for a number of use cases (as the reply and the event are
+typically within moments of each other), however many replies that existing users make are done
+so with no referencing context. For example, "does anyone know how to fix this?" means nothing if
+the original event's context isn't presented. This is largely an issue for bridges and other simple
+clients, though many projects which realize the reply fallback is not perfect tend to take action
+to support replies properly.
+
+## Security considerations
+
+https://github.com/matrix-org/matrix-doc/issues/1654 is still not fixed as part of this.
+
+## Unstable prefix
+
+Clients should use `org.matrix.msc0000` in place of `m.*` within this MSC. For example,
+`m.relationship` becomes `org.matrix.msc0000.relationship` while this MSC has not been included in
+a spec release.