>Oh don't be so vague, go on and name one!

See my reply above.

>I've been wondering the same thing...

Its not possible (or feasible) to implement the key telegram features in Matrix or other always-e2ee protocols.

>It might be easier to have some sort of plugin that overlays encryption on top of Telegram [...] and everyone needs to use your custom clients.

Its against the telegram api ToS. You are not allowed to implement features that require other parties to download your client. You can only add local features that do not affect other user who use the official client.