100% agree that this would be great
If they just did it as an "Alliance" feature so that you could pick any number of custom "alliance" chat channels that could only be joined by guilds (ie you specify that guilds x, y, z are in this channel) then imo it would be the best way (provided you can have multiple alliance chat channels).
This could be used in a number of ways -
1) A proper "Alliance" chat channel used by the full Alliances that are ingame
2) Any guilds that decide to "split" due to the new fame issues could have an extended guild chat
3) If the link to the chats could be made/broken at any time, the guilds that wanted to co-operate for a small amount of time (eg events etc) could use it as a planning/co-ordination channel for the duration
I don't think that any of these would reduce Around chat much; most of what would be put in these channels instead of Around would probably be more suited to a "private" setting anyway
As for FvF/GvG I think it would allow more tactics/planning/etc to take place without relying on the implemented faction channels... guilds that aren't in the same faction could then co-operate, or it would allow events such as the "peace dig" to be more co-ordinated
On the down side, I doubt that the chats could be made as temporary as my option 3 would require - they have shown that in the fact that the Faction channels do not disappear when you leave the area, but only when you log out. Even without this ability, I still think that options 1 and 2 are more than reason enough to make these channels, but if they do get implemented
please allow us to have more than one "Alliance" channel:
One channel has guilds A and X in it, and another has guilds A, X, Y and Z in it - the chat with guilds A and X in it would be an example of option 2 (extended guild chat), while the chat with A, X, Y, Z would be an example of option 1 (proper Alliance chat)