I also think that having "publicly available bots" that non-technical users could invite to their private room while maintaining decent privacy would be good. Hosting a bot may be trivial to readers here, but I think we can't reasonably expect all users to be able to set it up.
These all seem like things an XMPP server could choose to support. I don't think there's any standards work blocking such an implementation.
It could be implemented, but wouldn't it require breaking compliance with XEP-0045 by introducing a new send-only role, cf <https://xmpp.org/extensions/xep-0045.html#table-3>?
A more complex set up would be partially allow them to read messages directed to them and exclude the rest of messages.
MUC PMs would work for this, wouldn't they?
E.g: by specifying a command like /bot, the bot can read the content of that message to process it then reply back.
This sounds a bit too complex to me, especially when using E2EE. -- Nicolas