Currently you could do guild vs guild in WvW, but it’s not that efficient and fair since random players will join and interfere. And there is no way to keep score between guilds and rank them.
What you need is a GvG mode that let guilds to enlist a matchup in a tournament and rank guilds. Make it ladder (reset score history after a period) or whatever so that new guilds won’t suffer any disadvantage in the next ladder reset.
This mode will help existing/new players to know what guild is good to go if they are looking for coordinated group pvp instead of using map chat to ask people’s opinions about joining what guilds. Also, this mode fits more for the game’s name (GW2). :P
I bring this up since there are many complains about players imbalance in WvW. So a GvG mode can set fixed player counts for an even matchup between guilds. Also, there are so many random talks about what guilds are good on the forum. So let the score decide it instead of all the talks (guild pride rather than server pride). :P
Use something similiar to WvW but with smaller map just like spvp. Set a max player count for a matchup (for example 30) by guilds in their matchup option and maybe set a time limit for this matchup (say 1 hour). During the time of a match between the 2 guilds, their goals is to earn scores like WvW. Their guildies can show up anytime during this 1 hour matchup. Why not just go full 30 vs 30 at the beginning? Because this allows the option that some guilds might want to give handicap (well, they want to see if thier 20 guildies can win over 30. :P) Something like that. If Anet is seriously considering this GvG mode, it can design it anyway it wants that bring out max fun.