So once upon a time a designer had the idea to forbid player to speak to each others, provided they are enemies.
I do agree it's a nice concept to prevent harassment but well isn't it a bit too much?
Did you never wanted to congratulate the other team for winning or maybe harangue them :)
What I propose it let's keep factions separated as they are, but let players be able to speak to each others on a voluntary basis.
For example create neutral grounds taverns accessible to all characters where everyone can chat to anyone.
Let's take scenarios or battlegrounds: after a struggle you could check send me to the tavern option, warping you there. If anybody becomes too annoying you could right click on his name and squelch him/her for the duration of his stay in the inn.
And how about you could post adds like 'insert team name' challenge other teams in 'scenario name". There could be a prize like players putting a participation fee and well non participant could bet on the outcome.