My rough thoughts on this:
Moderation is too hard without servers. See the existing discussions on different group types. Though with the chatmail servers we might be able to add a protocol so that you can directly create and manage bigger groups based on mailing lists right from inside the delta chat interface. Those mailing lists are basically the server, so moderation and enforcing different roles is easier again.
Providing a separate mode to exchange the bubbles for a more compact discord / slack / rocket chat layout would be rather easy in comparison.
Also you can already look at the multi account feature as a way to have multiple contexts like discord’s guilds/servers.
About different conversations in groups, I think we should explore doing something similar to telegrams threading feature Threads in Group Chats - #10 by Simon.