Feature Request:
Add the ability to create subchannels within existing channels to better organize conversations and topics within a team and break large channels into focused topics.

Proposed Implementation: Follow the same pattern used by Posts with RootId. Add a ParentChannelId field to the Channel model that references the parent channel, enabling a single-level parent-child hierarchy. This mirrors how thread replies work - posts use RootId to reference their parent post, and we can apply the same approach to channels.

Link to Github issue: https://github.com/mattermost/mattermost/issues/33948