Here's some thoughts, using the idea of chevron arrows coming together to
exchange, both in a simple version, and one using the globe. The backgrounds are just placeholders, these are to show the logo idea rather than a completed banner.
Or with just the "X":
I also tried it with thinner chevrons and not intersecting, but I don't think it works as well:
Edit:
Incidentally, here's a suggestion for the forum icons with "useradmin" and "pm" matching the other three (and all four corners rounded):