Agree, this is an excellent enhancement, 
on 1-7.
I like this idea, and I do see value in being able to message users that I do not share a team with.
Similar to the "one direct message channel showing up across all teams" behavior, it would be great to have the capability for public channels to be able to show up on multiple teams - basically the ability to flag a channel as Global.
For example, you could have 3 large independent project teams, using their own Teams, but with a shared Water Cooler global channel between all 3 teams.