Proposal to allow overriding `m.room.power_levels` using `initial_state`

Signed-off-by: Tulir Asokan <tulir@maunium.net>

proposals/3214-initial-state-power-levels.md

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+# Proposal to allow overriding `m.room.power_levels` using `initial_state`
+Bots and bridges that create rooms often want to override the initial power
+levels. The spec defines a `power_level_content_override` field that is applied
+on top of the default power levels for this purpose. However, that field can't
+easily replace the entire default content, as it gets merged with the default
+levels.
+
+Synapse already allows replacing the entire power level event content by
+placing a power level event in `initial_state`. However, interpreting the spec
+literally means inserting the default power level event first, then inserting
+the one from `initial_state`, which might not be possible due to auth rules
+anymore.
+
+## Proposal
+The proposed solution is updating the room creation event order to specify that
+a power level event in `initial_state` must be used to replace the default
+content.
+
+Specifically, the third point in the [/createRoom](https://matrix.org/docs/spec/client_server/r0.6.1#post-matrix-client-r0-createroom)
+endpoint spec is updated to specify that `m.room.power_levels` events in the
+`initial_state` list should be used to replace the default power event:
+
+> 3. A default `m.room.power_levels` event, giving the room creator (and not
+     other members) permission to send state events. If the `initial_state`
+     list includes a `m.room.power_levels` event with an empty state key, that
+     event replaces the default one entirely. Otherwise, the
+     `power_level_content_override` parameter is applied on top of the default
+     power levels to create the final content.
+
+## Potential issues
+Server implementations will have to separately check for `m.room.power_levels`
+events in the `initial_state` list, which is slightly more effort than the
+current flow.
+
+## Alternatives
+`power_level_content_override` can technically be used to undo everything in
+the default power levels, but it's more convenient and intuitive if
+`initial_state` can be used to simply replace the first power level event.
+
+## Security considerations
+None, as this doesn't change any event authorization or state resolution rules,
+it just makes it more convenient to replace the default power level event.
+
+## Unstable prefix
+Probably not useful here, as there are no new fields and the behavior is
+already implemented in Synapse.