More channel types
Private channels will hopefully be implemented soon but I would like to see continued work on the capabilities of channels, here are some examples I think would be useful.
Hidden private channel - Each member is invited and none else can see it.
Visible private channel - Everyone can see the channel but only invited members can see content. Good if you want people to be aware that it exists so they can request access if needed.
Channel with external members - Like above but users not member of the team can be invited e.g. guests or people helping the team out in a single project.
Open channel - Everyone can seen and join the channel but none is added automatically. Good if it is only relevant to some members of the team or if you only need it sometimes then everyone can manage their own access.
Published channel - Like "Private channel with external members" but anyone can join the channel without being part of the team. Good if you want to be able to talk to the team to ask them for help as a group.
Shared channel - A channel that is part of multiple teams so that you have a common area for two teams to work together without having to create a third team that has the members of both teams.
Anonymous channel - A channel that anyone can access and ask questions in without even needing an account. Good for support or interacting with parts of organization that do not have access to their own accounts. With a channel like this you would likely want the option to embed it in any webpage maybe even as a chat for talking with customers online.
What do you mean "accomplished with existing functionalities", I would say none of these channel types exists since all channels always have all team members as it stands now.
Some could be done by creating a large amount of teams instead of channels but that is not a good workaround for these features and should hardly disqualify implementing them.
Disclamer I am not Microsoft employee. Andreas many of this can already be accomplished with existing functionalities, some of the things you are asking for are redundant, while others already exist but don't have "label" you are looking for. The whole point of UserVoice is to build functionalities that do not exist, but if we are asking them to build something that already exists then uservoice loses it's purpose and we start asking questions "why are you not doing this and that"...