I don't know if it possible to split the output of the server messages and players into separated channels but I would surely enjoy such a feature.

I am thinking like this:
/ch [option] [name]
where [option] could be:
join: to enter a channel
quit: to exit a channel
and a special channel named server a player should be able to leave to silence broadcasted server events.

Server messages to individual players should not be blocked by leaving channel server.

Factions + /gmsg

That's not really a good solution, as it requires players to always type a command. Also that doesn't allow for ignoring global chat, does it?

Well - we could add options / commands for such a feature of course.
But then I'd say it would maybe even be better to provide compat for one of the existing Chat-Tab mods so different groups would appear in different tabs or something like that...

We would have to make clear first what we want to achieve with this and make some proper plan.
Also I would not disregard that there are a lot of good chat mods already for MC and that it might not be actually worth to invest into a feature that others already can do way better...

That said, I'm all ear for suggestions.

I've been thinking about this request. The same algorithm of range in a modified form can be used to split chats into channels to multiple players (more than 2) can chat without other players can see this conversation.

Also, supporting client side Tabbychat as an example, channel prefixes would be useful in this scenario:
/fechannel join [channelname] <secret>
would the client able to receive all messages sent to the channel with the key while all other players won't. The argument <secret> is optional to join, but when a channel is made with <secret> it is required to be able to join that channel.

The client would receive "[channelname] bla bla bla". The brackets are open for debate but preferable to be instantly compatible with Tabbychat.

Things like topic are not needed.

Channels with secret should be persistent over server restarts.

Messages sent over key protected channels should not appear on server console unless config says otherwise.

Well - that's pretty much what I said you can do with factions.

A user needs to join or create a faction. That is a single channel to be used besides the general channel. No other channels can be joined when already in a faction. A user needs to leave one faction to join another to talk in that channel. Doing so the user will not receive new messages from a former faction/channel.

The mechanics are there but not useful for using factions as chat channels. Also, there is still no possible way to leave the server channel with general messages.