There are already what we consider the standard guilds. The hard core raiding guild. The raiding guild. The PvP guild. The causal guild. The leveling guild. The future is looking as if we have have a few new styles of guilds popping up soon. One is already beginning to form and another I do not see far behind, we are just waiting for the technology to catch up.
Meet the new guilds.
Looking for Raid Guild:
These are already starting to make an appearance. Guilds whos main focus is doing the looking for raid as a group on Tuesdays. Just like the hardcore guilds might do progression (or farm) on the reset, the LFR guilds do the LFR as a group on Tuesdays. Of course their main focus is their guild so everyone rolls need on everything and they all share it between themselves to get every one geared as efficiently as possible.
The idea for a looking for raid guild is a completely different type of play. It is to get as many people into a raid that has the "real" raid feelings while still being of the LFR difficulty level.
It is sort of like a leveling guild but for raiding. They would be leveling their raiding skills. They will do the real raid on Wednesdays and Thursdays but their focus is getting everyone geared through the raid finder and teaching people how to raid. They use vent, cauldrons of battle, food, and strategy. They actively use the LFR for its true intended purpose, a place to gear up, learn and have fun.
If the looking for raid ever comes out with a 10 man version I can tell you to keep an eye out for these types of guilds as more will begin to pop up. The looking for raid guild is the perfect step between the leveling guild and the casual raiding guild. It would give a guild the freedom to run the LFR or the real raid depending on who shows up.
It is quite possible you can see the looking for raid guilds also crossing paths with the raiding guild or even the hard core raiding guild when content is brand spanking new just to gear people up for the real raid a little faster. It seems like the looking for raid guild is something we will be seeing more of in the near future and seeing a hell of a lot more of should they ever make a 10 man version, like they should.
While many of those other types of guilds might do the looking for raid as a group you will be able to tell the difference between them by the fact the LFR guilds main focus is to make sure they do the LFR each week as early after the reset as they can.
Cross Realm Guild:
There are none of these yet and there will be none of these in the near future, but there could very well be some in the coming years if the feature is added and mark my words, it will be added. With the advent of cross realm raiding it is only a matter of time before cross realm guilds get added to the game.
Instead of having to pick and choose people from your real id friends list and assemble a group you will be part of a cross realm guild with them and others. This will make recruiting people for raiding guilds a million times easier. Now when you post you are looking for a healer on blackwater for your raid team you do not need to get people on blackwater or people willing to server transfer, you just need to get people willing to share their real id.
To make it so guild can still get guild achievements there will have to be a way to have people on different servers in the same guild as you so cross realm guilds can't really be that far behind. Can they?
The whole focus of guilds seems to be changing all the time as the face of the game chances. No guilds are just one thing any more, except for maybe leveling guilds but even they have some people that stay in them forever. Most guilds seem to be all encompassing now.
The cross realm guild however will be bound to what is currently considered high end content because they are on different servers. They will not run into each other in stormwind, they will not be able to help each other with quests or running alts through dungeons and things like that. They will be for raiding / battlegrounds / arena / dungeons, basically anything instanced. Does this mean that this will really be the future of hard core raiding? Get the best players from all over the world in one guild while being on different servers? This will change the face of world first forever if cross realm guilds come to be.
These are two types of guilds I can see popping up and in a way I think both would be really good for the game.