My suggestion is to allow private chat between multiple people (up to X, greater than 1).
I would suggest it be Invite Only (Right click -> Invite to Chat) to make it easier to use. People in chat could invite others to chat and kick people from the chat, exactly like a normal party.
My reason for the suggestion is I belong to a large guild, I have more than 1 person to talk to and I’m not always in a party with these people. It’s incredibly annoying to whisper more than 1 person and I think it’s impolite to have a personal conversation in an already crowded guild chat, much less S, M or T.
I don’t know with 100% certainty if Squad chat would be what I’m describing, but I don’t want to find out and be wrong for 100g.